The Oxford Companion to Wine

An absolutely terrific wine encyclopaedia, containing more minute information about wine production, history, enjoyment, and custom than anyone could ever want to know. The entries feel authoritative, the information is useful and eclectic. The editorial board does a nice job of escaping the hegemony of French wine - there's ample information on the rest of Europe (eastern, too), the US, South American, and Australia. If you really like wine, this book makes a handy reference. Note, though, that its intent is not to help you pick particular bottles of wine to buy. I'm still looking for a good timeless guide for that.