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 remember reading Watership Down twice in a row when I was little, and loving it even more the second time around. Read it a few more times when I was older, and now recommend it to almost everyone I meet…I love the thought he put into the rabbits’ myths and language!