I know this is a super old post, but I just thought I would update this a little, in case there's some other person out there in my place.

While Rob Collie's book is terrific, it's not a beginner book. Russo and Ferrari's book, PowerPivot for Excel 2010: Give Your Data Meaning, starts at the beginning, and they walk you through creating all the measures and calculations, which is a huge help. Maybe I'm dense, but I had to slow waaaay down and read it slowly. Definitely a book where you {read, build, test, modify, repeat}. For me, getting my head around evaluation contexts was a challenge, but once you understand that, it gets really interesting.