Paul Garland has at least three series of books that come to mind:
I Found A Box Of Photos Of My Wife... And Her Ex - A Hotpast Story
I Know Who You Did Last Summer - A Hotpast Story
I Visited My Wife's Hometown... And Found Her Hidden Life - A Hotpast Story
He's got plenty of other books that you might like, too.
There are also a couple of books by David McManus that might fit your description, Reluctant Cuckold. There's also a sequel, Cuck Storm Horizon, but I think the mystery part is pretty much "solved" by then.
Kenny Wright has a novel in two parts, In Too Deep, that could be worth reading. It's kind of a hotwife mystery thriller where a husband goes looking for his missing wife, only to be dragged into a dark and intriguing world of lies, danger and temptation.