The world is full of "bad books"; not just uninteresting, or ill-informed, or morally repugnant books, but books that set out to present or defend positions that are insupportable in logic. I speak here not of books such as Hitler's Mein Kampf but of books that include Erich von Daniken's Chariots of the Gods? (1968), which presents "proof" of visits to Earth by extraterrestrials, or of Barry Fell's America B.C. (1976), which "proves" that ancient Celts reached North America before the time of Christ, or The Holy Blood and the Holy Grail (1982), in which Michael Baigent, Richard Leigh and Henry Lincoln purport to prove that lineal descendants of Jesus (and his wife, Mary Magdalene) walk among us. The Holy Blood has the additional distinction of having been the inspiration for Dan Brown's best-seller The Da Vinci Code(2003). Often these bad books become quite popular, and frequently gain a wider audience than good books on the same subjects. In discouraging my students from relying on such bad books, I began to wonder why they are popular. Few are models of prose style, although most provide a brisk enough narrative. Most of them are long, between 300 and 500 pages. Are we seeing here just the literary equivalent of Gresham's law, or is there something else going on?