Birds
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For Britain and Europe, if you only get one bird guide, get this one - it's the best! It is available in softback, hardback and large format.
It is often the sound rather than sight of a bird that lets you know its around. Learning birds songs and calls adds considerably to the birdwatcher's skill and in some cases is essential for identification. This book comes with 2 cds which include recordings of most songs and calls you are likely to hear, together with helpful comments from the author.
For North America, I think this is the best bird guide. It comprehensively covers all species with excellent illustrations and distribution maps.