By pulp thriller standards, Code is brainy stuff: The standard car chases and intrigues are leavened with Crichtonesque discursions on medieval iconography, the formation of the early Christian church, and history's suppression of the ''sacred feminine.'' In other words, it ain't Clive Cussler. But it's not Umberto Eco, either: The story is a straightforward, code-cracking mystery quest for the Sunday Jumble set. Brown's recurring protag, Harvard symbologist Robert Langdon, is wrongly implicated in the murder of a Louvre curator, who leaves his colleague a cryptic message pointing to an ancient secret that could rattle the Catholic Church to its foundations. The cliches click pleasantly into place, like the tumblers of an old, easy-to-pick lock.