The Butterfly Conservation website
butterfly-conservation.org (contains an A-Z of British butterflies)
The Norfolk Butterfly Conservation Branch website
norfolk-butterflies.org.uk
The following two books contain both species accounts and site information:
Butterflies of Britain & Ireland, A Field and Site Guide by Mike Easterbrook (A&C Black 2010)
Discover Butterflies in Britain by D E Newland (WildGuides 2006)