Probably The Lord of the Rings. I read the entire thing almost ten years ago, and only remember bits and pieces. It's so long that I could definitely read it multiple times and still learn new information every time I read it.
I have the "Triology" as an old omnibus that I bought second hand. It's huge and very unpractical to read, but it does have all 5 parts in one book.
I'd take that or Steven Kings The Dark Tower series which I actually have as one long epub file that contains the complete series. That counts as one book too, right?
Long enough that by the time you were finished, you could return to the beginning and not get bored.
Alternatively, since it is so serialized, you could read individual chapters/sections and have a satisfying story conclusion. But nothing compares to the way all of the arcs come together for the final conclusion.
Either the SAS survival handbook because it'd be one of the most useful books to have completely memorized, or maybe the Bible just to dunk on fundamentalists by quoting obscure passages that contradict each other.
Neither sounds super fun, but those are the first that come to mind.
Eh, it's a mix. There are shorter, lighter cozy mysteries in there, but my favorite series is Karen Rose, and some of her recent books are stretching to 600 pages. There's a bunch of nonfiction psychology stuff when I can find new ones worth reading as well. (Behave is listed at 790, Thinking Fast and Slow at 499, as my 1A/1B favorites.) A lot is probably closer to 200-300 just by nature of what's available though.
Audiobooks at 2x speed 40 hours of work, plus driving, showers, stuff like yard work and other more physical projects all add up. It's not really an "achievement" as much as it is just valuing that content over music/radio/podcasts, and having a job where not having the stimulation of a book would make my head explode. (I do also have ereaders and use them, but most of the sheer volume is audiobooks.)
Edit: I kind of use goodreads? the goal is nonsense and I am really inconsistent with adding stuff. I try to do it in batches from my 50 apps eventually. It doesn't count any programming books, and I don't track multiple reads (so the several series I read again that I read in previous years don't count). But if you're curious those are the books I read for the first time this year. Availability on Libby or Everand are big factors, because I clearly can't afford to buy that many outright. I like a variety of different (author) voices, so I try to be open minded. I mostly prefer mysteries though.
If I had to pick just one book, it’d be "The Haunting of Hill House" by Shirley Jackson. It’s perfect for soft horror fans and really pulls you into its eerie atmosphere every time. Always a great read!
Maybe The Shadow Rising or A Memory of Light, but I’d prefer if I could have the whole Wheel of Time series.
Forgot to add the why. Because they are incredible fantasy novels with rich characters, fascinating world building, and a well-defined magic with a system of deference that I really like.
Also, The Expanse is a competitor in my mind. Different stories by miles but similarly worth reading on a loop.
If you suddenly haven only one book to read for the rest of your live, something big has probably happened to humanity and your interest in society would have to be historical not practical.
My first thought was to pick the longest book I know of, which is Harry Potter and the Methods of Rationality by Eliezer Yudkowsky. I think the PDF version is 2000 pages. Not really sure I want to reread it, though. So maybe the book I've reread the most times, which would be 11/22/63 by Stephen King. What a fantastic book.