Tor Doubles: Wrap Up: Other Doubles series
Now that I’ve looked at all of the official Tor Doubles, plus the proto-version and the unpublished version, where to next if you like the double format. Obviously, there are the Ace Doubles, which ran from 1952 until 1978. That series provides the reader with at least 261 additional volumes of science fiction, plus a similar number of westerns and numerous mystery novels. You can read Black Gate publisher John O’Neill’s thoughts about collecting and selling Ace Doubles in this article from 2017.