For more information about RAID, see the Disk-HOWTO, Module-HOWTO, and Parallel-Processing-HOWTO, available from the Linux Documentation Project and its mirrors (http://www.tldp.org/docs.html#howto).
For more information about clusters and high-availability setups, see:
High-Availability Linux Project, the definitive guide to load-balancing techniques: http://www.linux-ha.org/

Linux Virtual Server Project: http://www.linuxvirtualserver.org/

mod_backhand, which provides load balancing for Apache: http://www.backhand.org/mod_backhand/

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
Chapters 11 to 18 of Web Performance Tuning, by Patrick Killelea (O'Reilly).
Chapters 2 and 12 in Optimizing UNIX for Performance, by Amir H. Majidimehr (Prentice Hall).
Chapter 9 ("Tuning Apache and mod_perl") in mod_perl Developer's Cookbook, by Geoffrey Young, Paul Lindner, and Randy Kobes (Sams Publishing).