The Decompression Matrix by Bob Cole is a good introductory text on deco. It covers the traditional Haldane/Buhlmann approach but unfortunately doesn't cover any of the more recent or advanced developments in deco theory such as accelerated deco, bubble models, trimix, etc.
Beating the Bends by Alex Brylske is simillar in scope to Bob Cole's book.
At the other end of the spectrum Bruce Wienke's books are very in depth and cover the latest thoughts on deco. Bruce is the man behind the RGBM model. His books are very heavy on the maths and physics and are not the place to go for an introduction, but if you want to know the detals behind the models then they are very thorough.
Bennett and Elliott's Physiology and Medicine of Diving contains some good sections on deco theory but is not written for the layperson.
I have collected some useful decompression info and links on my site
http://www.dive-tech.co.uk/links.htm