Over the years I've worked with more Apache servers than I can remember, and will say without a shadow of doubt that this is single best reference I've ever used in regards to Apache.
The goal of the Apache Cookbook isn't to create a massive tome of information to sit on your shelf, but to some how magically contain an example for every odd configuration of the Apache web server you would ever come up with. More times than I can recall I've been looking around on the internet, trying to find an example of some configuration file only to have the lightbulb light up over my head and reach for this book. The answer is always there.
This book is an absolute must have for anyone who has anything to do with Apache servers on a daily basis, you will use this book eventually if you do, it's as simple as that.