Live Wiki Documentation
The Wiki Documentation will always be the most up-to-date.
The Web Application Defender's Cookbook: Battling Hackers and Protecting Users is a book written by the ModSecurity Project Lead and OWASP ModSecurity Project Lead Ryan Barnett. The book outlines critical defensive techniques to protect web applications and includes example ModSecurity rules/scripts.
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.
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.