ModSecurity Handbook: Getting Started Guide is
A free short book (about 100 pages) that consists of the first 4 chapters of ModSecurity Handbook. It contains everything you need to know to install and configure ModSecurity.
If you like the book, you may consider purchasing the full edition here.
ModSecurity 2.5 is
"A complete guide to using ModSecurity", written by Magnus Mischel. The book is available from Packt Publishing in both hardcopy and digital forms.
Apache Security is a comprehensive Apache Security resource, written by Ivan Ristic
for O'Reilly. Two chapters (Apache Installation and Configuration and PHP) are available as free
download, as are the Apache security tools created for the book.
Preventing Web Attacks with Apache. Building on his groundbreaking SANS presentations on Apache security, Ryan C. Barnett reveals why your Web servers represent such a compelling target, how significant exploits are performed, and how they can be defended against.