Not including short story collections, poetry collections, anthologies, etc.
For me, I believe it’s Oroonoko by Aphra Behn because it was on the syllabus for three different classes that I took in college. Other than that, the most I’ve read a single book is twice, and that was Telegraph Avenue by Michael Chabon. I’ve read Dracula 1.5 times. Didn’t finish it the second time around.
I’ve read The Dark Tower series 3 times and am planning a fourth with a friend this summer.
I’ve read Moby Dick 5 times (always skip the Cetology chapter).
The Hobbit many times.
Little Blue Truck, probably 1500 times or more.
Little blue truck, the true classic!