The Searching Dead — Ramsey Campbell

1952. On a school trip to France teenager Dominic Sheldrake begins to suspect his teacher Christian Noble has reasons to be there as secret as they're strange. Meanwhile a widowed neighbour joins a church that puts you in touch with your dead relatives, who prove much harder to get rid of. As Dominic and his friends Roberta and Jim investigate, they can't suspect how much larger and more terrible the link between these mysteries will become. A monstrous discovery beneath a church only hints at terrors that are poised to engulf the world as the trilogy brings us to the present day...