There are a number of books currently in print that deal with the Apache web server and its operation. Among them are:
Apache: The Definitive Guide, Third Edition (O'Reilly)
Apache Unleashed (Macmillan)
Apache Administrator's Handbook (Macmillan)
You can also keep an eye on a couple of web pages that track Apache titles:
http://Apache-Server.Com/store.html
http://httpd.apache.org/info/apache_books.html
