The Woman in Blue by Elly Griffiths

When Ruth's friend, Cathbad, is house-sitting for a friend, he sees what he believes is a vision in the graveyard next door. It turns out to be a young woman named Chloe Jenkins, who disappeared from a drug rehabilitation clinic nearby and is later found murdered in a ditch. DCI Harry Nelson and his team wonder who could have wanted Chloe dead, since she since wasn't a local resident. Then, Hilary Smithson, a female priest who is an old college friend of Ruth's, reveals that she's been getting threatening letters. Could there be some connection between the crimes?  In The Woman in Blue, Griffiths brings back Ruth, her friends, Nelson and his colleagues in another satisfying mystery.