The chain
(Book)

A parent receives a panicked phone call from a stranger who reveals that both of their children have been kidnapped by someone who demands that they abduct another child to prevent the murders of their own.

Your phone rings. You child has been kidnapped. And to free her, you must abduct someone else's child. Your child will be released when your victim's parents kidnap another child. Rachel will be part of the chain ... or her daughter Kylie will be killed. But the masterminds may have made a mistake this time. Rachel may be the one to finally break the chain .... -- adapted from jacket