Joint pseudonym. Audrey Kelley Roos [1912-1982] & William Roos [1911-1987] were a married couple who wrote about a a married couple; their series detectives were Jeff and Haila Troy, aided by NY Lieutenant George Hankins. Roos also wrote some non-series mysteries.
Final book in the Troy series which takes place 18 years after the previous title. The characters haven't aged, but have divorced. Yet, since one of the best parts of the series is the rapid-fire wit between the Troys, that's clearly missed. There are a few continuity issues, such a mention of Jeff's profession in the early books that is clearly incorrect.
Having said that though, the puzzle plot of the book is well-done and very carefully clued.