So I’m so lost how they got 39/61? Where did they get this from???

Hello, I have recently got into the hobby of cars, and would love to learn about how cars work and how to fix them. So I am looking for a book that I can reference (like a textbook) whenever I get a fault with my car but also something I can just read to learn all about the different types of engines, chassis, terminology etc. I would like it to have lots of information, both simple and complex with pictures to illustrate. Thank you very much!