Enhanced descriptions from Syndetics:

Sarah Fortune inherited her flat from one of her many lovers. Now a son has appeared claiming it is his, morally if not strictly legally, and he is using illegal means to persuade Sarah to give it up: abusive letters threatening her personal harm. As it becomes more difficult to ignore these missives, Sarah comes across Henry, a timid, lonely man whose upstairs neighbour is using every trick in the racketeer landlord's book to make him leave his home: litter in the shared hallway, continual noise, poison set out for his cat. It seems that if they swap accommodation for a while they may be able to deal with each other's problems. But these two strangers have unknown connections in common: a well-meaning widow, a struggling therapist, and a man who sets fire to other people's property for a living ...