Just finished reading this weighty tome from Ed Skoudis and Lenny Zeltser. Its a great book with 650 pages of good stuff which goes into detail about Worms, Viruses, Backdoors, Trojans, Rootkits and even naughtier things. If you haven't read it and you don't mind a good long book I can recommend it, despite the often heavy subject the writers are not afeared of a bit of malicious code and treat it with the right amount of wit. It's surprisingly easy to read and the subject matter is dealt with very well with equal balance for Windows and *nix malwares.
You can get a feel for the writing style at www.counterhack.net which is Ed Skoudis website and has some useful links over there.