For many readers, Jack Ryan embodies the essence of the modern American hero. Morally centered, disciplined, humble yet powerful, Ryan (and his onscreen incarnations in Alec Baldwin and Harrison Ford) has made Tom Clancy one of the most popular writers in the world. But as Clancy has constructed the Ryan mythology, he has quietly established Ryan's shadow double, John Clark. Appearing in The Cardinal of the Kremlin, Clear and Present Danger, and Without Remorse, Clark…