Good free guide for mod_security?
How about ModSecurity for Apache User Guide? It's a little dated, but I am sure it has plenty of relevant information.
It's not free, but the "ModSecurity 2.5" book is pretty good. Here's the Slashdot review: http://books.slashdot.org/story/10/01/13/1424213/ModSecurity-25?from=rss