If the rest of this book is about how to use mod_perl properly, this section is about what to do when things go wrong. It contains three chapters:
Chapter 21 explains what various errors from Apache, Perl, or mod_perl might indicate, and what you can do about them.
Chapter 22 is about what you can do to fix mod_perl problems at all stages, from configuration to compilation to runtime to shutdown.
Chapter 23 points you to various books, online documentation, mailing lists, etc. that can help bail you out when you're really stuck.