I haven't read these, but maybe they're what you're looking for?
The Kept Woman by Susan Donovan
Moving On by Anna Jacobs
And Mistress Makes Three by Francis Ray
The Spare Wife: A Novel by Alex Witchel
I've been thinking about making my own story into a novel... from a dark-humor viewpoint, I guess. If I can't laugh, I'd do nothing but cry... so I try to see the bright side of things.