Besides meticulous explanation of what a feature does and how it works, Peter Gasston also discusses the material at a higher level, giving you insight into why and when you would want to use certain features, and when there are multiple options which one to go for in your particular situation.
One thing I particularly like is the appendix at the end of every chapter pointing you to more reading material related to the chapter’s content. The book in itself is already very useful as a reference, but this makes it even more useful. Even though it works very well as a reference, I recommend reading it from cover-to-cover to get a good understanding of the possibilties. You can do that at high speed, and then later use parts as needed.