Congrats on completing the book
First of all congratulations. You've come a long way and you've successfully completed the book.
This chapter marks the end of our granite application and all of its features.
You must've learnt a lot of concepts via the process of building the application.
About the in depth chapters
We had set aside some content, which goes in-depth into certain Rails concepts.
The upcoming chapters deal with those concepts, and won't have anything to do with the granite application.
Reading the in-depth chapters will solidify your current Rails knowledge, and also will help you learn new advanced concepts.
Thus everyone should read the upcoming chapters too.
Happy reading and learning!