The book is quite an OK read. I would give it to younger developers or people who never worked with APIs before. For more experienced developers it has some inspiration here and there and sometimes can give you some valuable insight. Personally I found one or two things to think about. However, I could have survived without reading the book too with a little thinking myself.
The book could have been a bit shorter as it is now. Phil wrote it was edited and the style and grammar (from the view of a non-native speaker) is all well. However, some of the examples and explains are not "on topic". Also sometimes it's to deeply tied to PHP developers. A book on API design should not necessary explain how to use the Fractal PHP library. It should explain you need to take care on the data types, and that's it. At least, it does the latter, so I am happy.
In some cases I would have enjoyed a more deep dive in API design itself. So far it explained the concepts of REST very well and gave you some tools on hand to start. However this book doesn't help you that far you can build complex APIs. Maybe it was not meant that way, but I have understood it so it would help me to make great API I won't hate. Now I am able to avoid the most basic mistakes.
All in all it's not a bad book, but also one of which I expected a bit more. Again, beginners should give it a try, especially when they are working with PHP.
One final word, the humor used in this book is not for everybody. Sometimes it gave me a bad feeling, sometimes it was simply "too much" and repeating itself. Chuck Norris getting me? Phil getting me? Oh well...