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.
The junior novelization of Star Wars: A New Hope, because for a long time when I was little, it was one of the only few books I had. Luckily now I have over a hundred. As for what I’ve reread a lot more recently, A Christmas Carol, because I read it every Christmas. It’s an amazing book.