Apache is far and away the most widely used web server platform in the world, running on Windows NT as well as Unix and other platforms. It powers about half of the world's existing web sites and is still increasing market share. Apache also forms the basis of a number of commercial web servers, such as IBM HTTP Server powered by Apache and C2Net's Stronghold.
Apache is at the leading edge of web server development with many new technologies being implemented first. Apache has a flexible architecture that allows independent developers to add their own functionality by way of modules, written either in C or, with the advent of mod_perl, in Perl.
The Apache Pocket Reference, a companion volume to Writing Apache Modules with Perl and C and Apache: The Definitive Guide, 2nd Edition, covers Apache 1.3.12. It provides a complete overview of the command-line options, configuration directives, and modules, and covers Apache support utilities.
Andrew Ford is an author and consulting computer programmer who has been involved with the Web since its early days. His book Spinning the Web was the first on publishing with the Web. He also maintains a collection of quick reference cards at http://refcards.com.