The Doomsday Conspiracy is Sheldon's first experiment in science fiction. Most of his other books, however, involve mystery and international intrigue.

Parallels among his novels may be seen in secret organizations such as in The Windmills of the Cods (1987), which involves a terrorist organization that wants the Cold War to continue. The Sands of Time (1988) presents nuns from a Spanish convent on the run from a fascist officer who wants to kill them.

Sheldon's first novel The Naked Face (1970) was a highly praised murder mystery.