In her first book for middle-graders, the author of Show Me the Evidence (1989) offers a winning tale about a custody battle--over a dog. As ""Treasure,"" the springer spaniel in question belongs to Cricket--a lonely daughter of busy yuppie parents; as ""Coty,"" she was raised by Dominic, who lost her during his recent move to a housing development despised as ""Crackerbox"" by the girls in Cricket's neighborhood. A major complication is that, until Cricket and Dominic discover their mutual ownership of the dog, they are well on their way to crossing class barriers and becoming friends. The climax involves an all-out custody trial conducted in their fifth-grade class--and a subsequent hard choice between what is legal and what is right. Ferguson's depiction of Cricket's relationships with Dominic and with her distant but loving mother are believable and appropriately tough. The portrait of the class struggle is a bit more clichÃ‰d. Still, an original and involving story with well-drawn characters.