The Janus Stone by Elly Griffiths

Forensic archaeologist Ruth Galloway is called to a building site in downtown Norwich because human bones have been discovered. When the bones turn out to be from a child (missing the head), the police-- specifically DCI Harry Nelson--are called in. Later, bones from a cat, also without a skull, are found. Nelson and his team trace the property's ownership to wealthy Edward Spens and look up previous occupants of the home being demolished—a children's home run by the Sisters of the Sacred Heart, with Father Patrick Hennessey in charge. Father Hennessey reveals that a brother and sister did go missing in the 1970's and were never seen again. The police focus on finding out more about Martin and Elizabeth Black, the children who disappeared, and test the bones to see if they're from the right time period. Ruth, meanwhile, becomes more comfortable with her approaching motherhood, but discovers that someone is trying to scare her to death. Another enjoyable entry in the Ruth Galloway series.