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.
“Imajica” by Clive Barker
I guess I’ve read the translated german version twice or even trice in my teens (when my english wasn’t really that good) and to get the pure experience I read the original version just some months ago.