The Philanderer’s Wife by Katharine Trelawney

Paddy is the charismatic film producer who has got used to having his cake and eating it. Joscelyn has tolerated her husband’s philandering for years, and has always been confident that their unusual union is happy and secure.
But when one of Paddy’s girlfriends becomes pregnant all the relationships involved come under immense strain. Paddy has the competing demands of wife and mistress, and they come under threat from a third woman who is hoping to take both their places.
There are difficult decisions to be made, and this prompts a dramatic end in which the Philanderer’s Wife has to decide on the future of her marriage.