This article is from the Real Ale FAQ, by with numerous contributions by Brett Laniosh others.
A: The CAMRA "Good Beer Guide" is the premier guide to pubs that sell real
ale. Published annually, each county is given its own section. The pubs
are selected by local branches and there are natural biases towards
centres of population and easy to reach places, but most gems are properly
represented. The pubs are chosen by drinkers and the first criterion is
the quality of the beer. Like any other guide, it is not perfect but for
real ale enthusiasts this is probably the best guide there is. The GBG
includes a useful section that describes each beer and the breweries from
where they come.