It's twenty years old but it's still the best/practically only book of its kind: basically a giant, blisteringly dull and colossally thorough version of Robert Burton, but much more boring. Come on, guys, update this fucker. I've been rereading it since 1990 and I want at least a new typeface, if not a few handsome color plates. Oh, and you might finally join the DSM-V and give up and call it bipolar disorder, too.
EDITED TO ADD that I am a giant dork: the second edition came out in 2007.