The Crossing Places (Ruth Galloway Book 1), Elly Griffiths

Dr Ruth Galloway is called in when a child's bones are discovered near a prehistoric site on the north Norfolk salt marshes. Are they the remains of a local girl who disappeared ten years earlier - or are the bones much older?DCI Harry Nelson refuses to give up the hunt for the missing girl.

Since she vanished, someone has been sending him creepy anonymous notes about ritual sacrifice. He knows that Ruth's expertise and experience could help him finally to put this case to rest. But when a second child goes missing, Ruth finds herself in danger from a killer who knows she's getting ever closer to the truth.