Mystery Under Third Base, by Fran Orenstein
Eleven-year-old Willie, always the smartest kid in town, skips two grades, doesn’t have any friends, and spends his days reading in an abandoned old cellar. Suddenly, a boy who calls himself Huby as in the red ruby, appears out of nowhere and becomes Willie’s new friend. Who is this weird new boy who knows things he shouldn't, and might not be what he seems to be? Together they explore Willie’s favorite hideout where mysterious noises and dots of light could mean a ghost. When Willie wins a school contest with a proposal to build a baseball field, the project may fall apart because third base will be over Willie’s cellar hideout and whatever may be down there. With the help of new friends and the town witch, Willie solves the ghostly mystery under third base.