That's a good question, and a hard one to answer! But without doubt my favourite book on UK butterflies is Thomas and Lewington's "Butterflies of Britain and Ireland" which, unfortunately, is now out of print.

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I've recently got very interested in older books as well - E.B.Ford, Richard South, Frohawk etc. Absolutely marvellous texts!
I'd love to get hold of the 2 volume set of Frohawk's "Natural History of British Butterflies". But these are pretty hard to get hold of and expensive! But if anyone out there has a copy they want to sell, then I'd be very interested!
Cheers,
- Pete