This is definitely the SECOND step up the Power Pivot ladder. I was nearly driven crazy trying to research websites for a more basic explanation of the concepts in this book. I then found Learning to Write DAX by Matt Arlington which I now consider Step One in the Power Pivot ladder. The good news is both books use the same (or very similar) Data Base examples so they certainly flow well; you can step into Rob Collie's book without having to delve into the history of the ficticious data base company. If you know nothing about Power Pivot, then protect your sanity and start with Learning to Write DAX. After which, Power Pivot and Power BI will make so much more sense that it will be a valuable step up the Power Pivot learning ladder.
An interesting approach to building up my soft skills in knowing only pivot tables, I got this book for a reason wanting to know how to go further with monthly tables of data but got more than I bargained by having why this way was done by for former Microsoft member. Well done with thanks.
This is THE book to begin your next phase of Excel life. There is is pre-Power Pivot phase and a post-Power Pivot phase to your career. This is a great introduction to DAX and is worth re-reading over time as your understanding develops.
Couple this with Power Query and your power with Excel and Power BI sky rockets "M" is for Data Monkey is the must read book for Power Query.