Tor Doubles are a series of science fiction books published by Tor Books between 1988 and 1991, mostly in tête-bêche format. The series was inspired by the Ace Doubles, published between 1952 and 1973.
Titles in the series
This list is complete and includes ISBN numbers for the United States.
In 1985, Tor published two Keith Laumer novellas in the dos-a-dos format with the "Tor Double" label (The Other Sky/The House in November), but it was not part of the series.
At least one more in the series was prepared but never published: Esther Friesner's Yesterday We Saw Mermaids paired with Lawrence Watt-Evans's The Final Folly of Captain Dancy would have been series number 37.
References
Tor Double Novels Wikipedia(Text) CC BY-SA