An undemanding if unexceptional introduction to the heart, the circulation system, heart defects (and how they are remedied), and the causes and medical answers to heart disease in grownups. About this last, the Silversteins seem to soft-pedal the role of animal fats (""eating too much [unspecified] fat"". . . or ""too much sugar"". . . or ""too much of anything"" can lead to heart disease) and to play up the real advances in medical response. Still, they cover the basics capably, with a non-gimmicky presentation (""What is a heartbeat? . . . . Why doesn't blood flow back in from the arteries, the same way it went out? . . . . It might seem silly for the heart to have its own blood vossels"") that attends to reader interest.