Data analysis expressions (DAX) is the formula language of PowerPivot and this book is written to give hands-on practice to anyone who wants to become competent at writing such formulas. Sample exercises that explain each concept are provided and followed by practice questions and answers to maximize learning and experience with DAX.
One of the best books I have read. It was easy to follow. The explanations were very good and the examples and exercises helped consolidate the learnings.
I think Matt Allington's book around learning DAX is great. The way the book is written mimics if you are taking a class in a computer lab (hands on learning). Practice, practice, practice makes perfect with instant confirmation on what you just wrote in DAX!