To get started please check out:

The mod_perl guide,
   http://perl.apache.org/

Writing Apache Modules with Perl and C
   http://www.modperl.com/

the mod_perl developer's cookbook:
   http://www.modperlcookbook.org/

The Apache:: modules on CPAN:
   http://search.cpan.org/search?mode=module&query=Apache

... and of course the mod_perl mailinglist:
   mail modperl-subscribe@perl.apache.org < /dev/null