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.
Earth’s Children series by Jean Auel, The Chronicles of Narnia by C. S. Lewis, Otherworld Series by Kelley Armstrong and a very old favorite romance…The Wolf and the Dove by Kathleen Woodiwiss.