The most comprehensive guide to Belgian beers is probably the late Michael Jackson's Great Beers of Belgium. The only major weakness of this book is that Jackson seems reluctant to say anything negative about any beer, which suggests to me that his association with the Confederation of Belgian Brewers may have been a little too close for him to be regarded as a truly independent writer. Despite this, the book is highly recommended.

For "on the spot" information on drinking in Belgium, the best guide is Tim Webb's Good Beer Guide to Belgium which contains details of bars in every major (and many minor) town in the country, together with extensive brewery and beer listings. Tim also publishes Lambicland (with Chris "Podge" Pollard & Siobhan McGinn) which is a good guide to Lambics and the Payottenland bars in which they can be enjoyed. These and several other useful titles (such as Around Bruges in 180 Beers) are available from Books About Beer.

Classic Beer Styles: Lambic by Jean Xavier Guinard is a good read if like me you fall in love with this particular beer style.