This review covers a number of the theories behind the use of PBL in the classroom. The authors extensively review these theories so you get a more detailed sense of the design behind PBL rather than superficial explanations.

This is a newsletter from the Stanford University Center for Teaching and Learning. It details their experience with PBL. I particularly liked the description of the "ill-structured" problem used in their courses.

This website is very useful for those who are wanting instruction on how to integrate PBL into the classroom. It contains many resources including sample lesson plans, demonstrations, and sample problems.