The following books have been instrumental in developing my philosophy and as resources for problems and activities.
A Mathematician's Lament Paul Lockhart
A thoughtful examination of how a traditional mathematics education fails to show students the beauty, creativity, and playfulness of true mathematics.
What's Math Got To Do With It Jo Boaler
An alternative approach to mathematics education based on Jo Boaler's research and experience in implementing rich, student-driven problems.
Mathematical Thinking and Problem Solving Alan Schoenfeld
There are some amazing papers in this compilation, particularly Schoenfeld's "Reflections on Doing and Teaching Mathematics" and Romberg's "Classroom Instruction That Fosters Mathematical Thinking and Problem Solving: Connections Between Theory and Practice."
Thinking Mathematically Mason, Burton, and Stacey
Concrete analysis of the mathematical problem solving process and advice on how to cultivate these habits in students/people.
Fostering Algebraic Thinking Mark Driscoll
An excellent resource for problems along with analysis about the habits that the problems are attempting to foster.
Designing Groupwork Elizabeth Cohen
A great resource on designing group work, tasks, and fostering collaborative thinking.
Interactive Mathematics Program Key Curriculum Press
A progressive high school textbook series that focuses on problem-based units and a constructivist approach to daily class activities.