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.)