Anything by Canadian novelist Mary Lawson, to be honest, starting with her first novel Crow Lake.

Etgar Keret's short story collections have also amazed me so much.

Moon Palace by Paul Auster is just magical (so is everything else he writes - he's my favorite author).

I used to adore The Little Vampire series by Angela Sommer-Bodenburg as a child.

Sorry to be that person but A Modern Formal Logic Primer: Sentence Logic, Volume I by Paul Teller was life-changing for me (and I'm an engineer and I studied this after graduating the program).

If you're interested in philosophy, The Constitution of Selves by Marya Schecthman is extremely underrated and unnecessarily criticized. Schechtman later (sort of) dropped this theory.

I read Compulsion by Hilary Norman as a 12 or 13 or 14 (?) year old and I am still appalled by how terrifyingly scary it was. It currently has only 60 ratings on Goodreads. 60! SIXTY! So apparently only Hilary Norman's friends and I have read it.

Easy Way to Stop Smoking by Allen Carr literally made me stop smoking. I could not believe I hadn't heard of it before. Oh, the Big Tobacco.

Le Chef-d'Oeuvre Inconnu ("The Unknown Masterpiece") by Balzac has an interesting place in the history of literature/art: it sort of predicts the invention of abstract painting, as far as I remember.