Up until chapter 10, all the examples in the book were based on a single table, a sales table. Chapter 10 is about working with multiple tables that are joined together by relationships.
The chapter makes that point that, with PowerPivot, you don’t have to go through efforts to bring all your data into a single table to work with it and, in fact, it’s better if you don’t. A distinction is made between the table with the data in it (called the Data Table) and related tables that contain attributes that you want to use for analysis (called Lookup Tables). An example is given that shows how filter context flows across relationships. At the end of the chapter, Rob updates his diagram of how PowerPivot calculates values shown in the cells of a PivotTable. This model is key to understanding how to write effective DAX measures.
In addition to chapter 10, I also read through chapter 11 which is only half a page. In chapter 11, Rob makes the point that if you’ve made it this far you have already gained the skills to “improve the quantity and quality of insights you can deliver by 4-5x” and you have now gained the same amount of material as a full day of intensive training in one of Rob’s onsite, personalized courses.