Things you learn there is how REALbasic works behind the scenes. How the compiler handles your code and why things are like they are. Of course not everything is up to date as Aaron talks in a few articles about things fixed long ago, but as a REALbasic user for more than 10 years who knows a few of the bugs he mentions, it is very interesting to read why this bug existed and how it was fixed.
Another good story in the book you should read is how Aaron came to REALsoftware.
On the end I can say that I read this book this weekend and it was a pleasure. I think every serious REALbasic developer should simply read it to learn a few facts and to remember later where to lookup some things. I personally learned a few new things about the plugin writing