The Kingston Case

Back from their holiday on the Isle of Man, Janet and Joan Markham are ready to reopen the bed and breakfast to guests. Janet is surprised to learn that one of the guests who will be arriving soon is William Chalmers's former wife, Alice. But William, who runs a local antique shop, has bigger problems than that.

He's been receiving anonymous letters, telling him to leave Doveby Dale. While he has reasons for not wanting to involve the police in the problem, once he tells Janet about it, she insists on discussing the matter with Robert Parsons, the local constable.
Just because Robert is on the case, though, doesn't mean that Janet is staying out of it. William has several business associates who might benefit from driving William out of business.

Could one of them be behind the letters? Or could Alice be trying to get her former husband to move back to London? Can Janet deal with American visitors and Alice Chalmers at the same time and help William as well?