SICP. I've been seriously delving into functional programming with the aid of this great book. It's really refreshing, since FP shows you a different (and nicer) way of thinking about problem solving. On a related note, i've discovered a new treasure of Emacs' dragon cave: ILISP, a very useful package providing a nifty development environment for LISP languages (including Scheme/Guile, the one i'm currently using).
As an aside, i've been also re-reading the Feynman Lectures on Physics, just for fun and to avoid forgetting all that amusing things i once studied so eagerly (i guess i'm still a physicist, at least in spirit!).