The War of the Worlds is a novel by H.G. Wells about an alien invasion. In 1938 Orson Wells made it most famous when it was turned into a radio drama for an episode of "The Mercury Theatre on the Air." Because the drama was formatted as a series of newscasts, many people misinterpreted the broadcast and thought that there was a real alien invasion going on.
In "The Fright Stuff," Buster mentions this when he, Arthur, Binky, and Brain are trying to come up with a prank to pull on the girls at Muffy's party. Buster imagines a UFO landing with two aliens coming out to greet the girls, scaring them away.
In "D.W. Aims High," dad shows the book to D.W. to help her with her fear of Attack of the Sleepless Martian Dinosaurs by explaining that people have been afraid of aliens before, but they don't need to be, since they aren't real. (At least, the evil ones aren't real.)