The best place to learn about Apache's server-side include module, mod_include, is from the Apache documentation. The latest version of Apache documentation is always available from http://www.apache.org/. Look for information on Apache's modules and then look through the list for mod_include.
For some good tutorials on the subject (and some good ideas on how to use SSIs on your site), take a look at HotWired's Webmonkey site. They are listed under XSSIs at http://www.hotwired.com/webmonkey/backend/.
I was particularly impressed by their ideas regarding the use of timfmt to bring up different GIFs depending on the time and date.
