I think this book starts from the basics. It explains what is the purpose of an API, it explains how should an API be constructed, created and implemented. The book presents the known methods for building a web api. It is explained why apis which communicate through HTTP should be more widely used then the ones based on SOAP or REST or other protocols.
The firs half of the book really explains the details about constructing an api, presenting the possibilities and guidelines, which API developers should keep in mind. After presenting the general guideline, the book explains the structure of the ASP.NET WebApi, what is the execution flow of the API invokes, which class should be inherited when security is the primary concern and how are different filtering layers executed. It is presented how an ASP.NET WebApi application can be hosted inside IIS or ran as a separated process where there is no need for IIS.
There is a good explanation of the HTTP protocol, HTTP headers and the idea behind using HTTP as an overall communication protocol. The final book will contain explanations related to data formatters, the HttpClient implementation inside the .NET Framework and there will be a dedicates section for testing.
Another thing which was really sympathetic for me is the appendix, where the different media types are presented. Inside the book the author(s) put a big accent on standardization of media types and APIs. There are presented a lot of existing media types which are not so well known, but could/should be used when designig an API.
When I read the book it was available as a draft and also being a draft it had great knowledge and stuff in it. I would recommend this book to anyone who knows/plans to write an API which will be used by others and will need constant improvement and feature change.