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.
Ship of Fools by Katherine Porter. I’m not saying it’s the greatest book I’ve ever read, but the scenes while on the ship are so eloquently written, they’re inspiring, so I find myself reaching for it over and over.