I cannot stress how wrong I was. While certainly the book does focus in on embedding code in other web sites, the level of detail and coverage of various browser quirks makes this book invaluable to anyone.
This book is an incredible way to learn about how the DOM works, how script loading works, and really how the browser works as well. For every problem discussed multiple solutions are covered as well as a deep explanation of why you may choose one particular solution over another.
If you have no plans ever for building a third-party API, this book is still strongly recommended. Hell, the last two chapters alone on performance and debugging are worth the price of the book alone. This is a book I plan on keeping nearby as I feel like I'll be tapping the knowledge inside again and again.
You can read two samples chapters online now: Chapter 1, Chapter 4.
Here is the table of contents:
- Distributing and loading your application
- Rendering HTML and CSS
- Communicating with the server
- Cross-domain iframe messaging
- Authentication and sessions
- Debugging and testing
Finally, the book site itself: http://thirdpartyjs.com/