The Greedy Triangle is a most unusual book in that it will appeal to three age groups, 4-5 year olds, those learning polygons for the first time, and for adults who never felt that comfortable with geometry. The book opens up the reader's mind to seeing geometric shapes all around, while providing a simple basis to remember the differences among polygons (they each differ in having one angle and side more or less than the most similar polygon).
The book has an excellent guide in the end for parents, teachers, and other adults. This includes great exercises to extend this knowledge for your child. This section also explains the terms more precisely, and defines an undecagon (11 sides) and dodecagon (12 sides).