Found this in a dollar bin a while back. Listening to a man from the 70s describe starting a situationship by showing up to a tennis court with a broken racket is pretty entertaining so I do have to recommend it.
Good news though, he would keep a separate apartment to make it easier and no one can afford to do that these days so your significant other can't follow these tips anymore!
I found this among my father's books after he died and I knew that he almost always had a mistress over the course of three marriages. Since it was originally written in 1971, I expected to find the book somewhat dated and much of it is, but a good bit of it is timeless practical advice on the perils and pitfalls of keeping a mistress.