The Crossing Places by Elly Griffiths (Ruth Galloway #1)

A child's bones are discovered near the site of a pre-historic henge on the north Norfolk coast, and the police ask local forensic archaeologist Ruth Galloway to date them.

Are these the remains of a local girl who disappeared ten years ago? DCI Harry Nelson refuses to give up the hunt for this missing child. Ever since she vanished, someone has been sending him bizarre anonymous notes about ritual sacrifice, quoting Shakespeare and the Bible. He knows Ruth's instincts and experience can help him finally put this case to rest.

Then a second child goes missing, and Ruth finds herself in danger from a killer who knows she's getting ever closer to the truth...