Deadly stranger / Peg Kehret.

Enhanced descriptions from Syndetics:

When twelve-year-old Shannon is kidnapped by a psychopath, her new friend, Katie, finds her life in danger when she unknowingly becomes the kidnapper's only witness.

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Kirkus Book Review

A cliffhanger! Twelve-year-old Katie dreads starting at her new school, but it's not as bad as she expects when she makes friends with Shannon, who shares her interest in science and math. Suddenly, Shannon is kidnapped, and Katie is struck by a hit-and-run driver. Without realizing it, Katie has important evidence about the kidnapping. Ensuing chapters alternate between the two girls, as Shannon tries to outwit her captor and Katie gradually realizes that the information she possesses endangers her. In a nice twist, Shannon, having escaped, becomes the means of Katie's rescue. Both girls are courageous and resourceful--no passive victims here. They are appealing heroines, and their interest in something other than their male classmates is also refreshing. Credibility is occasionally strained, but overall this tightly plotted mystery is engrossing as suspense mounts at a satisfying pace: Shannon almost escapes, and is thwarted by her captor; and Katie finds herself in danger a second time. The violent circumstances may be somewhat alarming for younger readers, but this will be popular. Copyright ©Kirkus Reviews, used with permission.