By: Joshua Glenn

People Minus X (1958), by Raymond Z. Gallun.

By: Joshua Glenn

The Face of X (1960), by Lionel Roberts.

By: Joshua Glenn

Virus X (1945), by Sydney Horler.

By: Joshua Glenn

Commando X (1967), by Poke Runyon.

By: Joshua Glenn

The Man from Planet X No. 2: Tiger by the Tail (1975), by Hunter Adams.

By: Joshua Glenn

The Power of X (1968), by Arthur Sellings.

By: Joshua Glenn

Planetary Agent X (1965), by Mack Reynolds.

By: Joshua Glenn

Madame X (1966), adapted by Michael Avallone from the 1966 movie.

By: Joshua Glenn

Nurse Lily and Mister X (1961), by Diane Frazer.

By: Joshua Glenn

Echo X (1962), by Ben Barzman.

By: Joshua Glenn

Perry Rhodan No. 16: Secret Barrier X (1972), by W.W. Shols.

By: Joshua Glenn

Utopia Minus X (1966), by Rex Gordon

By: Joshua Glenn

Warrant for X (1945), by Philip MacDonald.

By: Joshua Glenn

The X Factor (1965), by Andre Norton.

By: Joshua Glenn

The Tragedy of X (1941), by Ellery Queen.