Apache User's Guide
New features in Apache 1.2
New features in Apache 1.1
New features in Apache 1.0
Compatibility Notes with NCSA httpd
Apache Reference Manual
Compiling and Installing Apache
Apache run-time configuration directives
Apache's handler use
Highly generalized API to server functionality
Using SetUserID Execution for CGI
Frequently Asked Questions list
How do I? documentation
for heavily loaded web servers.
The manual translated into Japanese is here.