Chapter 3 of Apache: The Definitive Guide, by Ben Laurie and Peter Laurie (O'Reilly), talks extensively about the Apache configuration process.
Chapter 8 of Writing Apache Modules with Perl and C, by Lincoln Stein and Doug MacEachern (O'Reilly), talks extensively about configuration customization with mod_perl.
The extensive configuration manual at http://httpd.apache.org/docs/.
mod_macro is a module that allows the definition and use of macros within Apache runtime configuration files. The syntax is a natural extension to Apache HTML-like configuration style. It's very useful if you have to configure many sections (e.g., when you have many virtual hosts) and haven't learned about <Perl>sections yet.
mod_macro is available from http://www.cri.ensmp.fr/~coelho/mod_macro/.