Jonathan Zdziarski (who is also the author of the excellent
DSPAM spam filter ) has written an excellent book:
ENDING SPAM. This is a paper book; but it's one of the better books on the details of spam and spam filtering I have seen. It even gets many of the CRM114 internals close to right.

. Here's another review.:
Ending Spam . You can get Jon's book via
O'Reilly or
No Starch Press
Robert Haskins and Dale Nielsen have written
Slamming Spam: A Guide for System Administrators . This book also has sections on CRM114. You can get it here:
Slamming Spam
Of the two books, I (wsy) recommend both enthusiastically; I'd say if you're more of a “I need to have deep knowledge of how does this actually work” person, go with Zdziarski first and askins/Nielsen second, and if you're more into “I'm a sysadmin, how do I hook this up in my server room”, go with Haskins/Nielsen first and Zdziarski second. The two books overlap about 50%, so it's worth reading both of them. Of course, that's just my (wsy) opinion, for what it's worth.
Shalendra Chhabra has released for public download his full master's
thesis from UCR. Shalen is one of the CRM114 developers, and has spent considerable time on integration of CRM114 with other systems. Here Shalen reveals a vision of “what's in store for the next few years”.
And of course the official guide: