SQL for Web Nerds by Philip Greenspun is great for starters and although it's written with Oracle in mind, one can learn a lot of general stuff. Another book that's online is Bruce Momjian's
Introduction and Concepts (I haven't read this one). Of course also check out the
PosgreSQL documentation and, maybe most important of all, lurking around this forum and those over at arsDigita has sure proven useful...
