The Secret at Haney Field by R. M. Clark

What do you get when you mix two baseball buffs— April and Darren—with a minor league team and a major league adventure? A grand-slam summer! Twelve-year-old April O'Day's summer has gotten off to a flying start. As the new bat retriever for the Harpoons, her hometown's minor-league team, she's fetching bats and doling out great advice to players and coaches alike. In a word, she's becoming indispensable. But mysterious things are happening at Haney Field, which April and her best friend—and fellow baseball enthusiast—Darren Plummer are determined to uncover. As they quickly learn, this is no ordinary season. In fact, it's a whole new ballgame!