Let X Be Excitement (1969), by Christie Harris.

Dangerous Mr. X (1929), by Francis Duncan.

A collector’s guide!

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.

The Big X (1959), by Hank Searls.

The Game of X (1965), by Robert Sheckley.

Venus Plus X (1960), by Theodore Sturgeon.

X (1963), adapted by Eunice Sudak from the 1963 movie X.

Tomorrow Plus X (1957 edition), by Wilson Tucker.

Earth Factor X (1976), by A.E. Van Vogt.