I wrote an article about Modsecurity, an apache module that work as an application Firewall. The paper is written in Spanish because the lack of spanish documentation about it. I wrote it for a monthly publication for the company i work.
You can get it at:
HTML Version: http://www.isecauditors.com/es/iseclab4.html
PDF Version: http://www.isecauditors.com/downloads/artic/iseclab4.pdf