The main goal of the ‘teaching mathematics through problem solving’ approach is to help students develop a deep understanding of mathematical concepts and methods by engaging them in problematic tasks in which the mathematics to be learned is embedded. This book aims to provide the coherence and direction teachers need to use problem solving to teach mathematics. An excellent resource which not only looks at the big questions but provides teachers with practical ideas to use in the classroom.
Teaching Mathematics Through Problem Solving is a good introduction to the topic with chapters contributed by John A. Van de Walle, James Hieber, and other leaders in the field of math education as well as 9 short essays by teachers describing their experiences with problem-based instruction. While it doesn't hold all of the "answers," I think this is a worthy addition to any elementary math educators library.