The Annotated Turing by Petzold (On Computable Numbers broken down, annotated, explained, and talked around. Would recommend to anyone interested in CS but without (yet) having formally studied it. I read it in sixth form, and it probably swung me more towards CS from my since-childhood intention of studying electronics. (Ended up doing a mix, but applied to some straight CS too.))
Theory of Computation by Sipser. (More subject specific and just a textbook really, I just thought it was a really great one and was able to read it ahead of my most relevant course.)
Sipser's book was the point I tried to give up pencile-pushing/parroting, before every proof there is a "Proof Idea" that really fills the gap between the theorems and proof, and the more important ideas you should take away, I suddenly realized every thing I learned had this discontinuity between things you do in problems and proofs, and the general understandings and usefulness and philosophy behind it.
Theory of Computation by Sipser. (More subject specific and just a textbook really, I just thought it was a really great one and was able to read it ahead of my most relevant course.)