5 Practices for Orchestrating Productive Mathematics Discussion by Margaret Smith & Mary K Stein "best book on how to turn your discussions from random and unstructured to equitable and meaningful" Amazon link here
Mathematician's Lament by Paul Lockhart "completely changed how I think about teaching math. Easy read"
Thinking Mathematically by J. Mason "Best book on problem solving that I've ever read"
The art of Problem Posing by S. Brown & M. Walter "Teaches you a structure for asking more questions and generating rich problems. I use it to create differentiated problems to keep everyone challenged"