What I'm reading now:

Prime Obsession: Bernhard Riemann and the Greatest Unsolved Problem in Mathematics, by John Derbyshire. I know--gasp!--two mind-improving books in a row? Perish the thought. I'm only part way through it, but it's exceedingly interesting and very well written; this says a lot, considering that the book is about a relatively dry subject. Let me share one amusing passage:

Published mathematical papers often have irritating assertions of the type: "It now follows that...," or: "It is now obvious that...," when it doesn't follow, and isn't obvious at all, unless you put in the six hours the author did to supply the missing steps and checking them.

This doesn't really have anything to do with Riemann or his hypothesis, but I spent so many hours in college trying to fill in these very missing steps from similar irritating lectures that I found the acknowledgement above pretty funny. Anyway, I highly recommend the book.