Samuel Youd, who died yesterday at age 89, was better known under the name John Christopher, one of several pseudonyms. And under that name, he was best known for creating the fearsome Tripods, alien invaders who enslave the human race with the mind-controlling "cap." Christopher's Tripod trilogy is a wonderful adventure story, but it's also a direct precursor to our current flood of young-adult post-apocalyptic/dystopian novels. He also wrote the ground-breaking eco-apocalypse novel The Death of Grass. The Guardian has a fantastic tribute to an influential, fascinating author who will be greatly missed.