A nice series of books for mathematical circles (both for students and for teachers!) published by the American Mathematical Society.

Mathematical Circles originated in Eastern Europe in the 1930s, spreading to Western Europe, Asia, and, eventually, to North America. Student circles are usually run by college professors, schoolteachers, or even enthusiastic parents wishing to share their appreciation of and love for mathematics.

The idea of teacher mathematical circles is more recent. Such circles are gatherings for elementary and secondary school teachers where they may develop effective approaches to teaching mathematics, with a particular emphasis on problem solving.