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.
Agincourt by Bernard Cornwell. It’s what got me into historical fiction and writing. Fantastic story, characters, and battle descriptions. You can feel the intensity in the right places and the way he describes combat is phenomenal