mod_perl documentation: http://perl.apache.org/docs/
http://www.modperl.com is the home site for this book, which is about creating web server modules using the Apache API. You absolutely must have this book if you plan to use mod_perl for anything other than speeding up plain CGI scripts. It will teach you the mod_perl API and provide lots of examples to learn from. This book is also very useful for developers who write Apache modules in C.
The mod_perl Developer's Cookbook, by Geoffrey Young, Paul Lindner, and Randy Kobes (Sams)
http://www.modperlcookbook.org/ is the home site of this book, which will save you a lot of precious development time. It provides out-of-box solutions to pretty much any problem or challenge you may encounter while developing mod_perl applications. Every solution is followed by an in-depth discussion, helping you understand how the solution works and making it easy to adjust the provided code to your particular situation.
mod_perl Pocket Reference, by Andrew Ford (O'Reilly)
http://www.oreilly.com/catalog/modperlpr/ is the home site of this book.
You should probably also get the Apache Pocket Reference, by the same author and the same publisher: http://www.oreilly.com/catalog/apachepr/.
See also Andrew's collection of reference cards for Apache and other programs: http://www.refcards.com/.
There are a few good books that cover technologies that deploy mod_perl. Among them are Embedding Perl in HTML with Mason, by Dave Rolsky and Ken Williams (O'Reilly), available from http://www.masonbook.com/; and Running Weblogs with Slash, by chromatic, Brian Aker, and David Krieger (O'Reilly). To see an updated list of books, please refer to http://perl.apache.org/docs/offsite/books.html.