Perl for the Web - Online book. The book provides tools and strategies to improve the performance of existing Web applications in Perl. Chapters 10 and 11 are relevant to mod_perl. - http://www.globalspin.com/thebook/
The mod_perl Developer's Cookbook - Companion site to the book, The mod_perl Developer's Cookbook. Information about the book, sample chapters and example code, and a list of resources. - http://www.modperlcookbook.org/
Writing Apache Modules with Perl and C - The home site of "Writing Apache Modules with Perl and C", a book about creating Web server modules using the Apache API, written by Lincoln Stein and Doug MacEachern. - http://www.modperl.com
Practical mod_perl - Book by Stas Bekman and Eric Cholet; full text freely available in HTML and PDF under a creative-commons license. - http://www.modperlbook.com/