Hi Chris,
I can heartily recommend:
Maximum Linux Security by Anonymous, published by www.samspublishing.com,
ISBN 0-672-31670-6
I wasn't to terribly impressed with it, large margins, big type, even worse the section on PAM sucked, there wasn't one.
As being a good grounding in some of the principles of linux security. The same author also wrote Maximum Security, which is a more general book, that I unfortunately haven't got yet!
You can get it on CD with a whole bunch of great (and old =) security software (I should know, I was the one that did a lot of the work on it, hah). ISBN-1-57595-0332-3 Again, I wasn't to terribly impressed with it, but it was a bit better then the Linux one. Another book that's recently out and pretty decent is: "Real world linux security" ISBN:0-13-028187-5 I edited it (so am pretty familiar with it =) and it's pretty decent in my opinion.
Iain
-Kurt