The Crossing Places by Elly Griffiths

When a child’s bones are found on a desolate beach in Norfolk, forensic anthropologist Ruth Galloway is called in by Detective Harry Nelson who believes the bones are those of a young girl who has been missing for ten years.  The bones turn out to be two thousand years old but Ruth is nevertheless drawn into the mystery of Lucy Downey.  Things get more eerie as another child goes missing and the detective receives bizarre letters alluding to ritual and sacrifice.  Ruth too is in increasing danger.  This is the first in the award-winning series about the acerbic yet likeable anthropologist.