I've never sorted out the authors behind the various Patrick Quentin, Q. Patrick, and Jonathan Stagge names, but I think I have the attribution above right. I've enjoyed several of the novels in the Peter Duluth series, so I thought I'd give this one a try. I'm sorry I did. In the other books I've read Duluth and his wife, Iris, have been pleasant company. In this one they're just two more unpleasant characters in a very unpleasant sextet. No other Duluth novels followed this one, and it's easy to see why.
Duluth, having returned from WWII with problems, can't get along with his wife, and they agree to a separation. She goes to Mexico, and he stays in New York, where he gradually returns to his old self. But it's too late. Iris has met a married man, Martin, in Mexico and fallen for him. The novel opens with Duluth at a bullfight in Mexico City, where he meets the man's wife, Sally. Things don't go well between the two, and they downhill from there.
The other two characters involved are Jake, a Hemingway-esque private eye (I saw a good many Hemingway echoes in the book), and Marietta, Martin's sister. It's hard to say who's more messed up, but my vote would go to Marietta.
Before the book ends, two of these people are dead, amidst hints of incest, plenty of homoerotic undertones, and much unpleasantness in vividly described Mexican locations. The local color is the best thing about the book, and the writing is fine throughout, so no complaints on that score.
The really bad news is that I had the solution pegged almost from the start, at least 100 pages before Duluth. It might have seemed novel in the late '40s, but it will be no surprise at all to a current mystery reader. Read any of the other Peter Duluth novels or stories, but skip this one unless you're a completist. Instead read the excllent new collection of stories entitled The Puzzles of Peter Duluth from Crippen & Landreu, which is highly recommended.