Chapter 9 ("Tuning Apache and mod_perl") in mod_perl Developer's Cookbook, by Geoffrey Young, Paul Lindner, and Randy Kobes (Sams Publishing).
The High-Availability Linux Project, the definitive guide to load-balancing techniques: http://www.linux-ha.org/.
lbnamed, a load-balancing name server written in Perl: http://www.stanford.edu/~riepel/lbnamed/, http://www.stanford.edu/~riepel/lbnamed/bof.talk/, or http://www.stanford.edu/~schemers/docs/lbnamed/lbnamed.html.
The Linux Virtual Server Project: http://www.linuxvirtualserver.org/.
The latest IPFilter: http://coombs.anu.edu.au/~avalon/.
This filter includes some simple load-balancing code that allows a round-robin distribution onto several machines via ipnat. This may be a simple solution for a few specific load problems.
The lingerd server and all the documentation are available from http://www.iagora.com/about/software/lingerd/.
The mod_proxy_add_forward Apache module, complete with instructions on how to compile it, is available from one of these URLs: http://modules.apache.org/search?id=124 or http://develooper.com/code/mpaf/mod_proxy_add_forward.c .
Apache::Proxy::Info, a friendly mod_perl counterpart to mod_proxy_add_forward.
Solaris 2.x—Tuning Your TCP/IP Stack and More: http://www.sean.de/Solaris/soltune.html.
This page talks about the TCP/IP stack and various tricks of tuning your system to get the most out of it as a web server. While the information is for the Solaris 2.x OS, most of it will be relevant of other Unix flavors. At the end of the page, an extensive list of related literature is presented.