A secret of the marsh

It is about a doctor who has always wondered about a place, called Frogs Abbey, which is located on a marsh, and its inhabitants. He was fascinated with it briefly as a child, but then he quits visiting the place it is located (Hempsal, in England). Some years later, he finds himself assigned to a job in that town and his curiosity is rekindled by an old shopkeep, Berry. He meets the man who lives at Frogs Abbey (John Wilmot) and his inquisitiveness regarding the house and its people is intensified. He then starts going there to see John Wilmot's "ill" mother, in order to make a bit more sense of the place. He then discovers many dark secrets the people of Frogs Abbey, and the marsh itself, have to hide.