To learn more about the Apache HTTP Server, refer to the following resources.
http://httpd.apache.org — The official website for the Apache Web server with documentation on all the directives and default modules.
http://www.modssl.org — The official website for mod_ssl.
http://www.apacheweek.com — A comprehensive online online weekly about all things Apache.
Apache Desktop Reference by Ralf S. Engelschall; Addison Wesley
Written by ASF member and mod_ssl author Ralf Engelschall, the Apache Desktop Reference provides a concise but comprehensive reference guide to using Apache at compilation, configuration, and run time. This book is available online at http://www.apacheref.com/.
Professional Apache by Peter Wainwright; Wrox Press Ltd
Professional Apache is from Wrox Press Ltd's "Programmer to Programmer" series and is aimed at both experienced and novice Web server administrators.
Administering Apache by Mark Allan Arnold; Osborne Media Group
This book is targeted at Internet Service Providers who aim to provide more secure services.
Apache Server Unleashed by Richard Bowen, et al; SAMS BOOKS
An encyclopedic source for Apache.
Apache Pocket Reference by Andrew Ford, Gigi Estabrook; O'Reilly
This is the latest addition to the O'Reilly Pocket Reference series.