A detective novel featuring a mystery writer who is the detective.

I have no information about this title, but I do have this about the author, Hulbert Footner:

"Footner's other series detective is Amos Lee Mappin, a successful, middle aged mystery writer whose crimes tend to occur in New York's cafe society. Mappin is unusual in that his "Watson" (at least in some of his tales) is a young woman, his secretary Fanny Parran. She is one of the few female "Watsons" in fiction, an example of how unusually female-oriented Footner's fiction was."