Walker Brown was born with a genetic mutation so rare that doctors call it an orphan syndrome: perhaps 300 people around the world also live with it. Walker turns twelve in 2008, but he weighs only 54 pounds, is still in diapers, can't speak and needs to wear special cuffs on his arms so that he can't continually hit himself. "Sometimes watching him," Brown writes, "is like looking at the man in the moon - but you know there is actually no man there. But if Walker is so insubstantial, why does he feel so important? What is he trying to show me?"

In a book that owes its beginnings to Brown's original Globe and Mail series, he sets out to answer that question, a journey that takes him into deeply touching and troubling territory. "All I really want to know is what goes on inside his off-shaped head," he writes, "But every time I ask, he somehow persuades me to look into my own."