I'm a software engineer with a long history of working with and building APIs. I bought the "RESTful API Design" book by Matthias Biehl to complement my extensive on-hand experience with a solid theoretical background and improve my conceptual skills.
The book covers nearly every imaginable subject related to API design: URL design, resources, relationships, authentication, authorization, and documentation.
While trivialities such as HTTP status code are covered in great detail, the author only scratches the surface of complex topics such as quotas, monitoring or performance optimization.
The book also covers two API description languages: OpenAPI and RAML. The respective chapters have pretty extensive code examples, but I could not see that one of these standards has major advantages over the other. I think covering one of these languages might have been enough. That being said, I skipped about most of the RAML chapter.
I found the most benefit for my own purpose in the early chapters, where the author shares his views about API portfolios and how a company's API portfolio should be considered a product of its own. Also, his insights about developer friendly API design being an integral aspect of an APIs success resonated well with me.
The attempt to cover everything is probably the main issue I see with this book. Also, there is a lot of repetitive content, which culminates in the same sentence appearing twice on a single page. Several paragraphs seem to have been copied, slightly modified and reused to work in different contexts.
While the book might be good for establishing a high level overview about API design concepts, I can not recommend it as a comprehensive resource to gain a deep understanding of that domain.