A quick tour of the things that make an API great. Pretty great as a reference, but shallow otherwise. The last two chapters deal with the actual design of an API, along with how to go about thinking of structuring the API. I wish there was more of that rather than what makes an API useful, as 99.9% of developers know why they're useful.
I would recommend this book to any programmer looking to design an API, or that's looking for a quick refresher of the world of APIs.
It is necessary to read to people who ask themselves the question: "Why do we need the API, who will use it and what will we get from it?" It very small book that has the right words.