The author motivates the design of the PPP protocol extremely well, and provides an intelligible roadmap to the RFCs and the vendor-specific "standards". Beyond that, he carefully points out shortcomings and misfeatures in various pieces of the PPP design and in various common PPP implementations, making the book far more useful and interesting than a "mere" reference on the subject would have been. (Not surprisingly, the shortcomings tend to exist in the proprietary PPP variants and implementations rather than in the open-standard and open-source ones...)
If your only reason for picking up the book is because you want to understand the difference between SLIP and PPP, you'll get more than you bargained for. But if you want or need to understand how PPP works, I can't imagine a better book than this one. The author guides the reader through the highly technical subject matter with satisfying authority. His delivery is strengthened by the exceptional editing and presentation of the publishers.