There are 1,000-page books about telephone systems, and there are 50-page reference pamphlets about a single protocol, like SIP. When writing a book like this, it's a challenge to strike a balance between the unwieldy tome that's filled with countless details of trivial import and a crash-course FAQ that tells you just enough to get started. On one hand, there's nebulous reference material, and on the other, there's not enough detail but lots of conciseness.
To address this, the book is rather detailed on certain subjects and rather brief and concise on others. If you're looking for more information in a special area, O'Reilly offers other books that may help you, including narrow but detailed books about T1 circuits, data communications theory, network security, and Ethernet. Safari Online is O'Reilly's web-based library of books, an invaluable tool that can also provide more depth. Information on O'Reilly's printed books and Safari are available at http://www.oreilly.com.