Crossing Places, The (Ruth Galloway #1) by Elly Griffiths; 2010. Softcover.

Forensic archeologist Dr. Ruth Galloway is in her late thirties. She lives happily alone with her two cats in a bleak, remote area near Norfolk, land that was sacred to its Iron Age inhabitants—not quite earth, not quite sea. But her routine days of digging up bones and other ancient objects are harshly upended when a child’s bones are found on a desolate beach. Detective Chief Inspector Nelson calls Galloway for help, believing they are the remains of Lucy Downey, a little girl who went missing a decade ago and whose abductor continues to taunt him with bizarre letters containing references to ritual sacrifice, Shakespeare, and the Bible. Then a second girl goes missing and Nelson receives a new letter—exactly like the ones about Lucy. Is it the same killer? Or a copycat murderer, linked in some way to the site near Ruth’s remote home?