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 need to finish the series . I read book 7 six years ago and still haven’t picked up book 8. Personally I felt the series peaked at book 2. The Great Hunt is one of my favorite novels ever and the other books just never lived up to it for me personally.