Apache's Home Page -- everything you could ever want to know about the server, including details on new releases and features
http://httpd.apache.org
The official Apache documentation, of which the tutorials and FAQ are highlights
http://httpd.apache.org/docs
More details on Server Side Includes than you can shake a stick at
http://httpd.apache.org/docs/howto/ssi.html
Book: "Professional Apache" by Peter Wainwright
An enormous and comprehensive tome, covering the entire Apache configuration file
Book: "Writing Apache Modules with Perl and C" by Lincoln Stein, Doug McEachern, and Linda Mui
An advanced guide to extending Apache's capabilities with your own code
comp.infosystems.www.servers.unix
The newsgroup where many Apache gurus and team members hang out
Weekly updates on the world of Apache -- news, reviews, even jobs
http://www.apacheweek.com
announce-subscribe@apache.org
An announcement-only mailing list listing releases, bug fixes and other information about Apache