*A Mathematician's Lament*by Paul Lockhart. Lockhart has an interesting perspective on mathematics and math education because he started out as a research mathematician and professor and then moved on to teach K-12 math.

For much of the book, Lockhart critiques the way math is taught in schools, likening it to teaching music only as music theory (never allowing students to actually listen to or play music until they are in college or even grad school) or teaching art by having children spend most of their early years simply learning paint application techniques and paint-by-numbers and not allowing them to look at real works of art or to free paint because they're not ready for such advanced material.