By: Joshua Glenn
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.
