I agree with Francesco- Griffiths and Arnold & Burton's books are pretty much essential. The new edition of Arnold & Burton is pretty much the same as the original one, but with a few taxonomic updates.
A couple of others you might consider are:
Beebee, T., & Griffiths, R., 2000. Amphibians and Reptiles- A Natural History of the British Herpetofauna
-Probably the best source of information on the three UK species (vulgaris, helveticus, cristatus).
The Great Crested Newt conservation handbook- available online at
http://www.froglife.fsnet.co.uk/HandbookF.htm (or free by post)
-lots of information relating to conservation of T. cristatus in the UK, very good if you're interested in knowing about T. cristatus in the wild.
If you're mainly interested in captive care, then the care sheets on Caudate Central are more useful than any book I've read.