At the Sound of the Beep

Ten-year-old twins Mathilda and Mathewstet run away because their separating parents plan to split them up. They intend to stay with an uncle in San Francisco, but he is away and the twins resort to sleeping in Golden Gate Park, where someone is murdering homeless people. The twins not only escape harm but they pinpoint the killer--an old woman who bakes poisoned angel-food cake for anyone she believes is hurting the park's birds. Then they rejoin their parents, who vow to try to keep the family together. Despite its gritty venue, this mystery/thriller is neither convincing nor suspenseful. It is weighted with contrived, implausible details, including the identity and modus operandi of the killer and the last-minute revelation that a battered woman the children have met (and bought food for) is actually an undercover cop. Neither the vagrants nor the twins and their family are fully realized figures, and uncertain pacing dissipates rather than builds tension. Ages 9-12. (Apr.)