Moths
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This is far and away the best guide for British moths. It has excellent illustrations showing several examples of species where tvariation can occur. The descriptions are the clearest I've come across, avoiding the jargon found in many other field guides.
Until recently, this was THE guide for UK moths but in my view it has been surpassed by the Lewington book above. Skinner is based on photographs rather than illustrations but the printing reproduction is disappointing. Also the specimens are pinned so are not shown in their natural resting pose. It can be useful to have a second reference book so if you are keen it is still worth having this on the shelf.
Though not a specialist moth guide, this has good coverage of the commoner species and when learning it offered me a quick identification route for many species without having to wade through the entire British list.