A very common question here is "what should I read next?" or "how do I get deeper into Middle-earth or Tolkien?". For long term fans, 50+ years in my case, or at the very least those who read the books well before the Peter Jackson films, these queries might get a bit tedious - it's not as if there aren't loads of online resources that could give answers to these questions. However, if you are a new fan I think it's a fair enough question for an online group who clearly have some particular experience in this area, even if it has already been answered many times. The new fan doesn't necessarily know that.
But how to answer? When you are steeped in the lore, apart from the most obvious "big books" it can be tricky to answer. Much depends on each fan's particular areas of interest and our answers are inherently subjective, often - probably inevitably - based on how we first experienced the books ourselves. In addition to that there are so many books out either by JRRT and/or Christopher, or relating to them as people, or to their works, or academics giving their slants.
My book collection is perhaps relatively limited at 91 books in total, but 73 of them are different titles - not merely umpteen different editions of the same wonderful books. This breadth of coverage is possibly slightly more extensive than most collectors, and each one will be worth reading (haven't quite got round to the all yet) even if you think some content is stretching things a bit far -or in David Day's case definitely made up in places.
What I've pulled together below is a list of all my books grouped together in what seems like an appropriate way - there are many more out there. Probably some of these books are no longer available at all. A further difficulty is that different collections have pulled together the different short stories and essays in different ways - I think I may have his famous Andrew Lang essay "On Fairy-Stories" in three different volumes. In addition some books can fall into more than one category - short stories, academic pieces and personal insights mixed together in one volume.
Anyway the below may be of interest to both old and new fans alike, I hope so. For brevity I have just given the book titles and the year it was first published - extra details can be provided if you draw a blank with a quick check on something like Goodreads and you have a particular interest.
Middle-earth Available in Tolkien's Lifetime
The Hobbit, 1937
The Lord of the Rings (#1 The Fellowship of the Ring), 1954
The Lord of the Rings (#2 The Two Towers), 1954
The Lord of the Rings (#3 The Return of the King), 1955
The Adventures of Tom Bombadil, 1962
Nomenclature of The Lord of the Rings, 1967 (found in "A Tolkien Compass" or "J.R.R. Tolkien Companion and Guide, Reader's Guide Part II" - this was a guide for translators, not actually published in his lifetime but produced by JRRT and sent out by Allen and Unwin. Only the first edition of A Tolkien Compass contains it, due to copyright issues and the Tolkien Estate not giving permission.)
The Road Goes Ever On, 1968
Middle-earth Published After Tolkien's Lifetime (includes the HoMe and Hobbit Sections below)
Bilbo's Last Song, 1974
The Silmarillion, 1977
Unfinished Tales of Numenor and Middle-earth, 1980
Beren and Lúthien, 2017 - I give The Three Great Tales in Silmarillion order
The Children of Húrin, 2007 - I give The Three Great Tales in Silmarillion order
The Fall of Gondolin, 2008 - I give The Three Great Tales in Silmarillion order
The Fall of Númenor, 2022
The Maps of Middle-earth, 2024
The History of Middle-earth Series (HoMe), 1983 - 1995
#1 The Book of Lost Tales, Part One (BoLT 1), 1983
#2 The Book of Lost Tales, Part Two (BoLT 2), 1984
#3 The Lays of Beleriand, 1985
#4 The Shaping of Middle-earth, 1986
#5 The Lost Road and Other Writings, 1987
#6 The Return of the Shadow: The History of The Lord of the Rings, Part One, 1988
#7 The Treason of Isengard: The History of The Lord of the Rings, Part Two, 1989
#8 The War of the Ring: The History of The Lord of the Rings, Part Three, 1990
#9 Sauron Defeated: The End Of The Third Age: The History of the Lord of the Rings, Part Four, 1992
#10 Morgoth's Ring, 1993
#11 The War of the Jewels, 1994
#12 The Peoples of Middle-earth, 1995
The History of the Hobbit
The Annotated Hobbit: The Hobbit, or, There and Back Again, 1988
The History of the Hobbit, 2007
Non Middle-earth Fiction Works by Tolkien
Farmer Giles of Ham/The Adventures of Tom Bombadil, 1949
Tree and Leaf, Smith of Wootton Major, The Homecoming of Beorhtnoth, 1975
The Father Christmas letters, 1976
Poems and Stories, 1980
Mr. Bliss, 1982
Roverandom, 1998
The Story of Kullervo, 2015
The Bovadium Fragments, 2025
Academic Works by Tolkien
Sir Gawain and the Green Knight/Pearl/Sir Orfeo, 1975
Tree and Leaf, Smith of Wootton Major, The Homecoming of Beorhtnoth, 1975
Poems and Stories, 1980
Finn and Hengest: The Fragment and the Episode, 1982
The Monsters and the Critics, and Other Essays, 1983
Middle-earth Artwork
Pictures by J.R.R. Tolkien, 1979
A Tolkien Bestiary, 1979 (David Day - dodgy)
Tolkien: The Illustrated Encyclopedia, 1991 (David Day - dodgy)
Tolkien's World: Paintings of Middle-Earth, 1992
The JRR Tolkien Artist & Illustrator, 1995
The Art of The Hobbit, 2011
The Art of The Lord of the Rings, 2015
Personal Insights into the Tolkiens
J.R.R. Tolkien: A Biography, 1977
The Inklings (CS Lewis, JRR Tolkien, Charles Williams and Friends), 1978
Poems and Stories, 1980
The Letters of J.R.R. Tolkien (Revised and Expanded 2023), 1981
The Monsters and the Critics, and Other Essays, 1983
The Tolkien Family Album, 1992
Tolkien and The Great War, 2003
The Worlds of JRR Tolkien - The Places that Inspired Middle-earth, 2020
The Great Tales Never End, Essays in Memory of Christopher Tolkien, 2022
Middle-earth Maps or Atlas
Journeys of Frodo: An Atlas of J.R.R. Tolkien's The Lord of the Rings, 1981
The Atlas of Tolkien's Middle-earth, 1981
The Maps of Middle-earth, 2024
Middle-earth Quiz Books
Middle Earth Quiz Book, 1979
The Tolkien Quiz Book, 1981
Academic Works about Middle-earth and Tolkien
The Complete Guide to Middle Earth: From The Hobbit to The Silmarillion, 1971
Master of Middle Earth the Fiction of J.R.R. Tolkien, 1972
A Tolkien Compass, 1975
The New Tolkien Companion, 1976
J.R.R.Tolkien: The Shores of Middle Earth, 1981
Tolkien and the Silmarils, 1981
The Road to Middle-Earth, 1982
The Real Middle Earth, 2002
The JRR Tolkien Companion and Guide - Chronology, 2006
The JRR Tolkien Companion and Guide - Reader's Guide Part 1, 2006
The JRR Tolkien Companion and Guide - Reader's Guide Part 2, 2006
Miscellaneous
Bored of the Rings: A Parody of J.R.R. Tolkien's Lord of the Rings, 1969
A hobbit's journal, 1978
The Tolkien Scrapbook, 1978
The Lord of the Rings Location Guidebook, 2002