Mystery, Solved
Blogenheimer, then Becca, two of my well-read friends, have found the book I was wondering about yesterday: The Dark Clue by James Wilson. (No, not the character from television’s House. Or is it? He does have an affinity for noir; see his office posters for proof.)
For bonus points, Weirleader came up with a more recent book, The Minotaur by Ruth Rendell writing as Barbara Vine.
Many thanks for your sleuthing. They’ll be added to my crazy-big “shelf” of books to consider in my library at Gurulib.