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.
Dirk Gently’s Holistic Detective Agency by the late great Douglas Adams who is more commonly known for his Hitchhikers Guide t the Galaxy series.
Though to be truthful, I’ve never read it. I have always listened to it on audio read by the author. My first encounter with it was an abridged version which was still quite decent, whoever abridged it did a good job. After the second Dirk Gently book came out unabridged, I had a nice letter exchange with his assistant asking about when the first book would get released unabridged on audio. Long story short, when the contract for the abridged version ran out.
Hitchhiker’s guide for me. The book and audio were a default for ages.
Try the red dwarf AUDIO BOOKS. Great time with that same British humor.
My SO got annoyed with me one day, because I was chuckling so much reading this book. I couldn’t explain why it was funny (I mean, you should just read it), so she couldn’t laugh with me. I cannot read this book around her.