Update 02/07/2017:

The book's original URL is no longer accessible, so I replaced it with a mirror website. This book was released under Creative Commons 2.0 license, so I guess this is OK.

Excertps from the Preface:

mod_perl is an Apache module that builds the power of the Perl programming language directly into the Apache web server. With mod_perl, CGI scripts run as much as 50 times faster, and you can integrate databases with the server, write Apache modules in Perl, embed Perl code directly into Apache configuration files, and even use Perl in server-side includes. With mod_perl, Apache is not only a web server, it is a complete programming platform.

Getting mod_perl running is easy. Tweaking mod_perl and Apache for the best performance and reliability is much more difficult. This book is about building mod_perl, using it, programming with it, and optimizing it.

Intended Audience:

To use this book effectively, you need to be familiar with the day-to-day details of running a web server, and you need to know the Perl programming language. We expect that you have already programmed in the Perl programming language. Having written simple CGI scripts and having experience with setting up the Apache web server are definite pluses. Knowing your way around the operating system is another plus, but not a requirement.

Reviews:

Amazon.com

:) "I would recommend this book to anyone who administers and writes for mod_perl, it fills the missing pieces in mod_perl Developers Cookbook and is a good companion volume to it."

:) "At almost 900 pages, this is certainly the biggest book on mod_perl that has been published so far. In my opinion it's also one of the most useful."