Spelunky by Derek Yu is probably the one that gave me the most bang for my buck when I was first really learning all the gamedev stuff

My obvious answer is Game Design Workshop by @kinojabber (imho the best book for learning game design)

My off the road answer is Game Design: A Brainstorming Toolbox by @dperry (has a bunch of lists that are helpful for when I'm brainstorming things)

I like this one

Seconding this! Spelunky has a great mix of coding/ game design theory, and introspection into the game design process and the weird mixed feelings that come along with making games. Such a good book

I've been reading A Theory of Fun by @raphkoster. The first book of game design I'm reading but loving it so far!

I'm going through this one right now, and it's definitely been helping me out!

designing games by @TynanSylvester is one of the best I've ever read. there also used to be a great one that cane with a crappy first person fame engine but I haven't been able to track it down

i think "Designing Games" by @TynanSylvester gets really close to what game design is supposed to be

(note that there's two books, he later released a newer book on game design called 'chris crawford on game design'. the original is called 'the art of game design'. i prefer the original but both are good)