I just picked up the Snort 2.0 book by Syngress. So far its good. Though I have lived and died by the snort docs from snort.org for the last few years, this book is a much needed, and more indepth approach to setting up and understanding the IDS. Really, I've only so far read the preprocessors chapter, but still, from the little bit of browsing I've done through it, I reccommed it.