Yesterday I read Symmetry and the Monster, (link goes to a detailed review by John Walker) which nibot recommended to me. It's about the classification of finite simple groups: one of the biggest, if not the largest and most involved project in all of mathematics. It's an okay book, I guess, but it's kind of light-weight, and I actually didn't find the high-level explanation of the nitty-gritty mathematics all that clear. Anyhow the discussion of just how weird and tantalizing "Monstrous Moonshine" is was definitely good. I wish there was more to say about Borcherd's proof and what (if anything) it reveals about why the connection is there.