Seeking a children's/teen book I read in the 90's, the central character is a boy who went to a pond/lake when he was told not to and developed warts on his hands as a result. These are cured by a magician? in an early scene, a friend of his father maybe? I only remember two more scenes, one in which he remembers trying to pull a wart from his skin, and another in which he dreams? of a giant chessboard. The white pieces are standard king, queen, knights etc, but all of the black pieces are crows. Crows and chess pieces were a recurring image in the book, but I don't remember what purpose they served. Thanks to anyone who helps with this, it's stumped me for years!