The Crossing Places by Elly Griffiths
Human remains are found buried in the peat in Saltmarsh near Norfolk. Archaeologist Ruth Galloway is called in to determine if the remains are ancient or if it could be Lucy Downey, a five-year-old girl who went missing ten years ago. DCI Harry Nelson hopes that he has finally found Lucy and is disappointed to learn that the body is that of a young girl, but a girl who was sacrificed during the Iron Age. Threatening letters were sent for a year when Lucy first went missing and seem to link her disappearance to Saltmarsh and the henge there. When another young girl, Scarlet, goes missing and letters once again start to arrive referencing the area DCI Nelson goes to Ruth for help puzzling out the mystery.
This is one of the few instances I’ve read where the expert in the field (amateur sleuth) and the police detective work well together. Ruth doesn’t go off and try to solve the mystery. She trusts that DCI Nelson knows what he’s doing, and he does. This is not to say that our amateur sleuth doesn’t find herself in some interesting situations, but we can forgive her because she didn’t do anything silly to end up in peril, she remains intelligent and true to character throughout.