Murder In Black Letter by Poul Anderson is becoming even more expensive as a second hand paperback but has also become available very cheap as an ebook so I might be reading and posting about it shortly.
I once looked in a mystery specialist bookshop, exactly like an f&sf specialist bookshop except for the obvious difference of subject matter. Isaac Asimov's detective novels and Black Widowers collections and Anderson's Trygve Yamaura novels and detective short stories would be in there but not their other works and probably not Asimov's Elijah Baley novels or Wendell Urth short stories, Larry Niven's Gil Hamilton stories, Anderson's "The Queen of Air and Darkness" or "The Martian Crown Jewels" or Anderson's and Gordon R. Dickson's "The Tale of the Misplaced Hound."
I think it is fair to say that sf mysteries are read by sf fans, not by mystery fans. Years ago, there was an sf sports comic which I venture to suggest was read by sf fans like me, not by sports fans like many of my contemporaries. But I am all for any genre-mixing that our creative types may devise.