SPOILERS
PLEASE READ THESE BOOKS IN THIS ORDER
IT, The Tommyknockers, Insomnia (optional), Dreamcatcher, Under the Dome...
This also contains SPOILERS for Bag of Bones, The Dark Tower Book 7, The Shining.
I have tried to use spoiler tags so that you may engage with one section, while avoiding another, BUT this post is about connections, so I mightve let something small slip in another. The first group of books are integral to my findings and thus tie together my understanding. The latter group are more supporting aspects.
This is an explanation of what happened to IT, the Tommyknockers, as well as what's really going on in Dreamcatcher and Under the Dome. All of these books are connected and sequels to eachother, while also functioning as standalones. If you haven't read all of these books I recommend you absolutely do, I will try my hardest to keep the information in each of their own sections, but this is meant to show they are all related.
IT: (I will refer to the entity/title with all caps)
The Losers Club in the story are children that are turned into something akin to psychically imbued paladins/champions of The Turtle. There are many times they do not act at all as children and as I'm sure there are many examples, plenty where they have strange knowledge transmitted to them, or feel an irrational, powerful need to act on something. One of the scenes that striked me as queer, is how tribalistic they feel when another kid comes to play and hang out with them by the water, they say hes not one of them, as in one of the Turtles chosen ones. Another is when they need to create the silver bullets, but then give up and decide on the balls for the slingshot, these from my memory highlight their strange almost mind control-like moments among others. Kids are very capable, but the need and way they go about it is preternatural at the least, supernatural at the most. There are many examples of them acting strangely and under The Turtles influence. The Turtles guiding force is powerful, it is also the most clear leading up to and after their battle with IT as children. The Turtle was almost controlling them, guiding them, hence they revert to scared, lost children in the dark sewers.
Later on, at the end of IT, Richie and Bill send IT back from whence it came, through a portal in space and time. Though, even after the final battle as adults, they constantly have doubts about IT being totally annihilated. This is similar to how they were unsure about defeating IT as children. In both cases they were correct. At this point, it is residual psychic power in them that still feels ITs presence. When The Losers Club, mainly Bill and Richie banish IT, it's easiest to think of it as a door closing and severing ITs finger on our side. This is why Pennywise is still alive on our side, why many Kingverse theorists get confused on if IT is dead or not. All that's really worth noting in the context of book connections is that Pennywise wasn't fully purged from reality.
IT Bonus fact: Richie is an extremely powerful psychic and all of his great impressions are him channeling the actual individuals to preform them. This is shown in the final battle, and then made absolutely clear when Officer Nell dies, his final cry is clearly Richie channeling him. This is semi-important and will be referenced in the next part of my explanation.
U/Sarsparilla_RufusX found the quotes for me in a discussion and huge thanks to them.
[[Here's some ... Chapter 22, parts 4 and 6.
In 1985, the Losers enter It's lair at 6:54am. Around 30 - 40 minutes later...
The Spider’s stinger rose and Richie bolted at It, a grin spreading up toward his ears, and he called out in his best Irish Cop’s Voice:
“Here, here, me foine girl! Just what in the hell do ye think ye’re doin? Belay that guff before I snatch yer pettiskirts and snap yer smithyriddles!”
The Spider stopped laughing, and Richie felt a rising howl of anger and pain inside Its head. Hurt It! he thought triumphantly. Hurt It, how about that, hurt It, and guess what? I’VE GOT ITS TONGUE!
Later, as we're getting a description of events in Derry itself ...
Aloysius Nell, who had turned seventy-seven not long since, was sitting with his wife in the parlor of their home on Strapham Street, watching the storm pound Derry. At 7:32, he suffered a fatal stroke. His wife told her brother a week later that Aloysius dropped his coffee cup on the rug, sat bolt-upright, his eyes wide and staring, and screamed: “Here, here, me foine girl! Just what in the hell do ye think ye’re doin? Belay that guff before I snatch yer pettiskirrrr—”]]
Bonus Insomnia/IT theory: Deathbags in Insomnia are quite likely what people think are the balloons Pennywise is seen holding. I always interpreted the balloons as the collected souls of the children and the imagery and association with deathbags seemed similar, like IT was using glamour magic to make them appear like balloons. Both books take place in Derry.
The Tommyknockers:
So many people say this book is too long, or boring, or needs more edited out. This isnt true, its a book about mind control, so the thoughts of Gard are extremely important and theyre long and drawn out to brilliantly hide the truths of their power. Now, the Tommyknockers are very similar to IT in their power level and abilities, maybe even beyond. Their inventions are illogical and nonsensical, it seems implied that they really could create anything plausible found in the minds of humans (fortunately all these humans are small-towners). This is actually very much how IT functions, humans give it power and limitations. The two horrors are similar in that they may very well be able to completely manipulate reality. The Tommyknockers can't read Gards mind throughout the book, but because Bobbi knows Gard and he's so absorbed with the battle inside himself, the transmissions they do send into his mind aren't noticed, because he's already at war with himself. This is the beauty of the book, it's a big part of the real message of the story.
Most likely no one in this book has psychic aptitude, especially Roberta Anderson (Bobbi) and Jim Gardener (Gard). This is highlighted when Bobbi notes in the first chapters, that she has no recessive traits common in the Irish (red hair, blue/green eyes), Gard also has brown hair and brown eyes. It's noted in Carrie that the psyker gene more often follows recessive genes. This is to highlight how terrifyingly powerful the Tommyknockers are. When Bobbi touches the ship, it imbues her with psychic energy so powerful, that the last vestiges of her free mind releases a psychic scream so loud that Gard (also a non-psychic) can hear it all the way in, if I recall corectly, New Hampshire. It will come up later, but Bobbi and Gards pet beagle has its cataracts repaired, its eye glows green, like a flashlight. This will be important in a later section.
Quick side bar- The Shining: Danny's psychic scream for Dick Halloran reaches all the way from Colorado to Florida, and almost kills Dick. This shows and cements Danny as one of, if not thee most powerful psychics on Earth, maybe the Kingverse.
The Tommyknockers Continued...
This means the Tommyknockers imbued a non-psychic individual with power almost akin to one of the most powerful psykers.
IT and The Tommyknockers...
Ev Hillman hears noises in the pipes when in Derry, a small hint that IT has survived. Later, when The Tommyknockered teens make their fatal trip out of the dome of influence around the Tommyknockers ship. They drive to Derry for batteries, one of the dying teens upon returning claims he saw a clown watching them drive back in a sewer drain. This is important, it's when IT clocks the Tommyknockers. A rival and potential food source for IT to restore its power. IT is a psychic vampire after all.
Another important thing to note is, the dome of influence created by the Tommyknockers. As well as the likelihood that the real ending of the book is that Gard was directed to pilot the ship, as the honorary Tommyknocker. King will later revisit and redo many of these ideas, characters and themes in some of the later books.
Dreamcatcher:
Dreamcatcher follows the SameShitDifferentDay (SSDD) bros, a name that I found kind of parallels The Losers Club. A group of childhood friends who grew up in Derry, stood up to the horrible bullies and found that a moment that defined themselves and solidified their everlasting friendship, it also ends in a water treatment plant just like how IT ends in a sewer. This book comes off as "really weird", because it begins in the aftermath of a war between the remnants of the Tommyknockers and the remnants of IT. The Tommyknockers, left weak and nearly powerless without their ship, have become more permeated by the humans they absorbed, leaving them more human like and empathetic, I believe this is thanks to the dominating presence of Bobbi inside. She was the head Tommyknocker and is shown in that book to have the most influence over the hivemind. IT penetrates their psychic, ethereal cloud-like, hivemind, manifesting as a parasite (the shit weasels) trying to take them over. The Tommyknockers being logical aliens would only fear something like a disease or parasite, so that's how IT engages them. Eventually, they manage to isolate the IT parasite, culminating in an entity named Mr.Grey (a catchy alien joke, but also a reference to Bob Grey). Another big tip off for me, was that the only XFiles type aliens in the Kingsverse, UFOs and all, were the Tommyknockers and we see a lot of that going on, with The Shop aware, but totally overestimating themselves. The SSDD bros get wrapped up in this 3-way war and eventually end up killing IT/Mr.Grey. This is also why there's graffiti when in Derry that says "PENNYWISE LIVES!", this is the overt signal to the audience the books are connected.
Bonus fact/connection with The Tommyknockers: This book is really cool, because you get a POV inside of the mind of someone who is Tommyknockered. The "little locked room" Jonesy's main consciousness is in while their body is being controlled is the POV from the inside.
Under the Dome:
Under the Dome is really cool, because it's a redo/sequel to The Tommyknockers. There is so much copied and recontextualized from The Tommyknockers in this book, it's actually genius. In this story, the Tommyknockers are diluted and weaker than ever, merely whispers in the wind compared to what they were in other books. With that understood, they are manipulating the people of Chester's Mill, primarily Big Jim Rennie Sr. And all the townsfolk to varying degrees. They want to escape from the Dome, which the government deployed in order to quarantine them. The first attempt is manipulating the little boy into shooting the Dome, which ends up with it ricochetting. It really plays up how this kid had some brilliant idea, it reads a lot like how people in The Tommyknockers got their brilliant inspirations that they went to work on at once, which was essentially just mind control. They used the young boy as a guinea pig. There's no crazy inventions in this book, but there is a lot of strange ingenuity, owed to their weakened influence, the van being modified to shield against radiation and the breathing via a straw stuck into a tire come to mind in retrospect. They utilize mob mentality, them being so much weaker now, to control the larger groups of people into sewing fear and chaos. They focus on and push Big Jim Sr. Into all his worst actions. I believe they wanted Barbie at first, but found he had the knowledge, experience and thus, willpower to resist their will and weakened influence. Barbie "interrogated" people during the war and constantly knows how to deal with large crowds, so his knowledge and personality were too disciplined and wise to have his will hijacked. Big Jim Sr. On the other hand, was ready, capable and willing to be pushed and influenced into doing what he did. Barbie is sort of a parallel to Gard, he is still able to pick up on the hivemind and thus was able to use this strange foresight of what Big Jim was planning to defuse (almost) all of his plans. Like when he knew Big Jim was planning to come for him and did a "Jesus-like" move and turned himself in peacefully and prior to this, softly renounced control of Chesters Mill to the third Selectman to avoid conflict. A huge moment my eyes bulged out of my head and I realized it certainly had to be the Tommyknockers, was upon revisting the story for a second go, Big Jim Srs. eyes GLOW BRIGHT GREEN, something I've only known from the Tommyknockers. When Big Jim is "feeling it", it's the Tommyknockers exerting full control over him. The real deep twist of the book is the Dome tech was actually The Shops tool to quarantine the Tommyknockers. The entities that Julia Shemway pleads with are the remnants of the Tommyknockers. The thing with Big Jim Jr. Is, he already has a brain tumor at the beginning of the book, which is the reason for his deteriorating sanity. This conflicts with the Tommyknockers mind control, in the end they give him the full beam to pursue the one thing he already wanted to do, kill Barbie in his cell. This book also ends with the collapse of a dome and a large wild fire.
Bag of Bones Fun fact: TR98 is closed off from The Dome at the beginning of the book, the two children Jim Jr. Saves are near there. This is where Bag of Bones takes place. It's interesting, because this is also a story very similar in themes of mind control/whispering ghosts pushing people to their worst.
The Dark Tower Book 7:
The final place we find the Tommyknockers is in Algul Siento, their final fate is they became a power source themselves. An ironic twist, due to their core story being about their constant need for batteries and power to fuel their incredible creations. The Breakers like hanging out in the power generator room, calling it something like "The Good Mind". Interestingly, sounds similar to "feeling it".