This book covers Vonage-type services (phone-centric product) and Skype-type services (computer-centric product). It goes into some depth into the pluses and minuses of both kinds of service, and lists some of the competitors.
The problem I had with this book is that so much of the content is time-sensitive: features, price, issues. It would make a great web site with all VOIP service offerings up-to-date and comparable, highlight current issues (such as some ISPs delibertately blocking VOIP), and the all-important customer feedback.
For addressing an interesting technology in (my opinion) the wrong media, I give this book a 6 out of 10 - a good read, but will soon be outdated by in the rapidly evolving voip world.