I imagine a lot of the information here is going to go out of date fast, but if you are looking at playground books today, here is where to start.

1. The WWDC 2016 talk, Introducing Swift Playgrounds, has a lot of technical detail about the .playgroundbook format and how the LiveView proxying works.
2. The demo for that talk is available on Apple’s dev site, but it’s out of date. I’ve been updating it for each iOS 10 Beta (to track Swift 3). Go to my GitHub to get the fixed Talking to the Live View sample.
3. The documentation for the .playgroundbook format can be found on the Apple Developer site.
4. Erica Sadun, who has written the book on Xcode playgrounds did some early investigations of iOS Swift Playgrounds. You might want to read them — here is Part I, Part II, Part III, Part IV,  and Part V.
5. Ash Furrow made a playground book linter and authoring tool (source). You can use the linter on any .playgroundbook (however you make it), and the authoring tool allows you to specify the parts of the book in a .yaml file, and it puts each piece in the correct file and folder for you in the .playgroundbook package.