Steranko paintings graced the covers of this Shadow massmarket paperback series that ran for 23 volumes between 1974 and 1978.

From the back cover:

WHO KNOWS WHAT EVIL LURKS IN THE HEARTS OF MEN?

It began with a murderous game of cat-and-mouse played on trains hurtling between London and Paris. When the game was over, three men were dead, and the terrible truth was clear: the most critical military secrets of the Western powers were in the hands of the legendary Parisian archcriminal, Gaspard Zemba!

The police mission: find and destroy Zemba. But His henchmen roamed every street and alley in Paris, yet he himself was so cunning, so elusive that no officer of the law had ever seen him, much less apprehended him.

Here indeed was a master of darkness, without peer in the world. Except one. One whose astonishing skills at moving by night and disguising his person were employed in the service of good, not evil. One whose lightning-fast hands wielded the blazing automatics of justice, not crime. One whose implacable pursuit was the despair of villains everywhere--The Shadow!