I've never seen so much Apache specific security information in one place! I've scanned through the entire book, and only read a few chapters so far. To actually review my current Apache setups and implement appropriate changes will take considerably longer, but should be well worth the effort.
There are both direct (i.e. Apache configuration) and indirect (i.e. PHP) tips and strategies, and numerious links to specific Apache security topics. This is a meaty, almost 400 page book, with little fluff. Both Apache 1 and 2 are covered. An online resource is also provided that includes examples, tools, links, and more.
Since I'm in the process of setting up a new webserver, I'll be keeping this book by my side each step of the way as I do so. This book is highly recommended. (This book is also available as part of O'Reilly's Safari online book program.)