There are so many excellent books on wine that I find it extremely difficult to compose an effective short list. Furthermore, books on wine tend to have the nasty habit of going out of date relatively quickly. The wine world is, after all, changing so rapidly these days.
That said, Michael Broadbent's Pocket Guide to Wine Tasting packs a great deal of worthwhile information into a small and easily browsed format.
In lieu of book recommendations, however, might I instead recommend a subscription to Decanter Magazine? This way, you'll remain abreast of the most up-to-date information, and experience the added benefit of regularly reading articles penned by some of the best wine writers around.