Apache Guide: For More Information ... Page 3
Although a little dated, this book from O'Reilly is a great guide to Apache, written by Ben Laurie and Peter Laurie.
This little book, by Andrew Ford, covers all the basics, and is a great thing to carry around once you are already familiar with the basics.
There are a ton of resources available to help you learn about the Apache server. With this list, you should never be left wanting more information.