In the tradition of the old Ace Doubles (flip one book over to read the second), here's the twentieth Wildside Double:
THE ASTEROID MURDER CASE: A Science Fiction Mystery, by Arthur Jean Cox.
The murder of UN observer Carl Neal on a lonely outpost of the Asteroid Belt would seem to be only a small human tragedy, and yet it opens up vistas both of millennia of time and of unimaginable distances. Inspector Tom Dooley, Chief of Security of the American Sector of the Belt, together with his faithful "Watson," Ralph Phelps, must sift through the many clues to unravel the mystery of Neal's death--and then keep the aliens from interfering. A first-rate science fiction adventure combined with a classic mystery!
A COLLECTOR OF AMBROSES AND OTHER RARE ITEMS, by Arthur Jean Cox.
Four tales of SF and fantasy, including the title story, in which three unscrupulous book collectors encounter a true "Completist"; "The Boy in the Iron Mask," which touches on the contamination of reality by dreams; "The Slaves of Moxon and Other Tales of Scientific Romance," in which the SF world has to deal with an alternate history version of...itself!; and "A Reply to L. Ron Hubbard." Great stories by a master storyteller.
