Heartwood Mountain: The Adventures of Wilder Good #8

When thirteen-year-old Wilder and his friends Sunny and Corndog go down to the river for a horse ride, something panics Corndog into running away. Wilder and Sunny reluctantly return home without him, and the next day a strange man comes to Wilder's house looking for Corndog. He seems to know an awful lot about the small, red-haired, foster kid's past. So Wilder, Sunny, and another friend, Big, set out to find Corndog before someone else does. If they succeed, can the group of friends convince Corndog to trust the world enough to rejoin it?