The Denver of the Secret Nine is an admittedly tortured misuse of a fictitious book title or predicament that P.G. Wodehouse used in several stories.
From Mating Season:
And that, of course, was that. It was no good telling her that I would prefer not to touch young Thos with a ten-foot pole and that I disliked taking on blind dates. When Aunt Agatha issues her orders, you fill them. But I was conscious, as I have indicated, of an uneasiness as to the shape of things to come, and it didn’t make the outlook any brighter to know that Gussie Fink-Nottle would be among those present at Deverill Hall. When you get trapped in the den of the Secret Nine, you want something a lot better than Gussie to help you keep the upper lip stiff.
And Heavy Weather:
‘Too risky. You don’t know what that house is. There’s Lady Constance after the thing and Gally Threepwood after the thing and Ronnie Fish and … well, as I said to Monty Bodkin this afternoon, a fellow trying to smuggle that manuscript out of the place is rather like a chap in a detective story trapped in the den of the Secret Nine.’
For the moment, it’s the provisional name of the Denver chapter of The Wodehouse Society.