Apache.org main documentation:
http://httpd.apache.org/docs/2.2/

Apache core features:
http://httpd.apache.org/docs/2.2/mod/core.html

Apache configuration directives:
http://httpd.apache.org/docs/2.2/mod/directives.html

Apache Bug search:
http://issues.apache.org/bugzilla/query.cgi
http://www.gossamer-threads.com/lists/apache/

.htaccess files provide a way to make configuration changes on a per-directory basis.
http://httpd.apache.org/docs/2.2/howto/htaccess.html