To learn more about the Apache HTTP Server, refer to the following resources.
http://httpd.apache.org/ — The official website for the Apache HTTP 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 weekly newsletter 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 the Apache HTTP Server 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 the Apache HTTP Server.
Apache Pocket Reference by Andrew Ford, Gigi Estabrook; O'Reilly — This is the latest addition to the O'Reilly Pocket Reference series.
Red Hat Enterprise Linux System Administration Guide; Red Hat, Inc. — Contains a chapter about configuring the Apache HTTP Server using the HTTP Configuration Tool and a chapter about configuring the Apache HTTP Server Secure Server.
Red Hat Enterprise Linux Security Guide; Red Hat, Inc. — The Server Security chapter explains ways to secure Apache HTTP Server and other services.