I have their cookbook and 30-day book but rarely use them (I can't eat grains, dairy or soy, so most of their recipes don't work for me), and everything in the 30-day book is explained more thoroughly in PP & PPLP. And I like thorough - knowing WHY & HOW it all works is what keeps me on track.

I regularly refer to both PP and PPLP as well as Slowburn, their exercise book with Fred Hahn.