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 & Lewington. This is available in a field guide paper back and a handy concise format that is ideal for field use.

And........
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;

Checklist of Lepidoptera recorded from the British Isles J.D.Bradley
Microlepidoptera of Europe Umberto Parenti
British Pyralid Moths Barry Goater
British and Irish Pug Moths Riley & Prior
British Moths and Butterflies A photographic guide Chris Manley
British Plume Moths Colin Hart
Nationalnyckeln Lepidoptera: Micropterigidae - Psychidae
Nationalnyckeln Lepidoptera: Roeslerstammiidae - Lyonetiidae
The Moths and Butterflies of Northumberland and Durham (part one & two) Dunn & Parrack
The colour identification guide to Caterpillars of the British Isles Jim Porter
Tortricidae of Europe (vol one & two) Jozef Razowski