As mentioned in the article, the mod_perl home page is at http://perl.apache.org/. This web page contains pointers to the most important documents and programs relating to mod_perl, including FAQs, a mailing list and the latest releases.
In particular, the “how to tune mod_perl for maximum performance” FAQ is quite useful; you can read it at http://perl.apache.org/tuning/.
Little printed documentation is available for mod_perl. Installing mod_perl does install a number of manual pages for mod_perl and for each of the individual Perl packages that come with it, including Apache::Registry and Apache::Status. If you would prefer printed copies of those manual pages, they appear in the second volume of Perl Module Reference, which is part of the Linux Resource Kit and published by O'Reilly & Associates.
All listings referred to in this article are available by anonymous download in the file ftp://ftp.ssc.com/pub/lj/listings/issue51/2974.tgz.