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.
Either Camus’ The Stranger or Stevenson’s Dr Jekyll and Mr Hyde.
I love novellas, they’re my two favorites. I read them both very young originally, and made efforts to read them again as I have gotten older because my perspective has changed. I can happily say that each read through experience is different, and that every decade or so it’s surprising which of the two my perspective has changed more from last time or over time.