Just one book
If you are looking for a book to help identify moths then the one essential book we would recommend is Moths of Great Britain and Ireland Waring, Townsend and Lewington. This is available in a field guide paper back and a handy concise format that is ideal for field use.

The next book we would recommend is the excellent Colour Identification to Moths of the British Isles Bernard Skinner. This again covers the macro moths and includes excellent plates showing underwings that makes it an excellent identification tool.

Thereafter there are a number of publications which we have found helpful;

A field guide to the smaller British Lepidoptera, A.M. Emmet
Bird-dropping Tortix Moths of the British Isles, Clifton and Wheeler
Checklist of Lepidoptera recorded from the British Isles, J.D. Bradley
Conifer Moths of the British Isles, Clifton and Wheeler
Microlepidoptera of Europe, Umberto Parenti
British Pyralid Moths, Barry Goater
British and Irish Pug Moths, Riley and Prior
British Moths and Butterflies a photographic guide, Chris Manley
British Plume Moths, Colin Hart
Field Guide to the Micro Moths of Great Britain and Ireland, Sterling, Parsons and Lewington
Identification keys to the Microlepidoptera of the Netherlands, Kuchlein and Bot
Nationalnyckeln Lepidoptera: Lasiocampidae - Lymantriidae
Nationalnyckeln Lepidoptera: Micropterigidae - Psychidae
Nationalnyckeln Lepidoptera: Roeslerstammiidae - Lyonetiidae
The Moths and Butterflies of Northumberland and Durham (part one and two) Dunn and Parrack
The colour identification guide to Caterpillars of the British Isles, Jim Porter
Tortricidae of Europe (vol one and two) Jozef Razowski