Mastering Regular Expressions, by Jeffrey E. F. Friedl (O'Reilly).
Chapters 2 and 4 in Operating Systems: Design And Implementation, by Andrew S. Tanenbaum and Albert S. Woodhull (Prentice Hall).
Chapter 4 in Modern Operating Systems, by Andrew S. Tanenbaum (Prentice Hall).
Chapters 7 and 9 in Design of the UNIX Operating System, by Maurice J. Bach (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).
The Solaris memory system, sizing, tools, and architecture: http://www.sun.com/sun-on-net/performance/vmsizing.pdf.
Refer to the Unix Programming Frequently Asked Questions to learn more about fork( ) and related system calls: http://www.erlenstar.demon.co.uk/unix/faq_toc.html.