By: Joshua Glenn
People Minus X (1958), by Raymond Z. Gallun.
The Face of X (1960), by Lionel Roberts.
Virus X (1945), by Sydney Horler.
Commando X (1967), by Poke Runyon.
The Man from Planet X No. 2: Tiger by the Tail (1975), by Hunter Adams.
The Power of X (1968), by Arthur Sellings.
Planetary Agent X (1965), by Mack Reynolds.
Madame X (1966), adapted by Michael Avallone from the 1966 movie.
Nurse Lily and Mister X (1961), by Diane Frazer.
Echo X (1962), by Ben Barzman.
Perry Rhodan No. 16: Secret Barrier X (1972), by W.W. Shols.
Utopia Minus X (1966), by Rex Gordon
Warrant for X (1945), by Philip MacDonald.
The X Factor (1965), by Andre Norton.
The Tragedy of X (1941), by Ellery Queen.