At 12, Hummer yearns for a horse--not only because her pony's getting old, but because nothing is right in her life: her obese, mentally ill mother sits sucking her thumb in a house full of garbage, while her father, unable to cope, has escaped to the barn. In the meantime, Hummer is ridiculed at school despite the lies she tells to protect her family. Then Hummer catches a stray Arabian mare, and the owner--Riley, a crusty old rodeo-cowboy--befriends her, lends her the horse, and trains the two of them for a long-distance endurance ride. Hummer is a believable blend of hopefulness, confusion, and growing self-confidence, but Gruenberg's real focus here is the training and competition--with plenty of authentic details garnered from her own experience as a horse trainer. The mother's mental illness is sensitively described, but the events after Riley's eventual intervention--Hummer goes to a foster home, from which she immediately runs away; a few days in an institution give hope of the mother's recovery--seem contrived and implausible. Still, a promising first novel that will appeal most to horse-lovers.