The Blackmoor Bones. The Seaside Sleuths

Two summers ago, in Pebble Cove, three young friends found an empty box beneath a crooked lighthouse. One summer ago, on a beach at Sandyhook, they found a stone with a single letter carved on it. And now, in the third summer, a letter has arrived from a place none of them have ever heard of - a tiny village called Blackmoor, three hundred miles inland, on the edge of a wild grey moor. The third sailor's family is waiting.
The last box has been hidden for a hundred and eighty-four years. And someone else - patient, polite, and quite without scruples - has been waiting too. Three children. Three old women. Three old houses. One last secret. And a story that has been trying, for a very long time, to come round. The Blackmoor Bones is the third and final book of The Seaside Sleuths, a quiet coastal mystery series for readers age 7 and up.
Friendships are tested, an old story is finished, and three young detectives discover that endings, like beginnings, are worth taking your time over.