Paul,

Although I get a tad exasperated with his frivolous style all of the time, I would suggest that the best book is Rob Collie's book

I have the eBook, and whilst I might claim I didn't really learn anything from it, I have been using PowerPivot for 2 years, have read the Russo/Ferrari book, and present a course on PowerPivot, I would still concede that it is probably the best starter.

He doesn't cover data modelling to any extent, and I argue that this is a crucial aspect, especially when trying to use PowerPivot to link Excel sheets, I have had some real fun and games with tables that are many-to-many relationships.