Hmm. The roots of this book are pretty obvious. Some of the basic plot lines have been done much better elsewhere. For instance: Young woman who has been educated in cryptography - Neil Stephenson's The Diamond Age. Dubious proceedings involving a plane flight from France to London - Agatha Christie's Death in the Clouds. Half-baked ancient lore - Baigent, Leigh and Lincoln's Holy Blood, Holy Grail. Bad guys who are masters of disguise and preposterous conspiracies - Agatha Christie's Tommy and Tuppence. Half-baked ancient lore combined with bad guys who are masters of disguise and preposterous conspiracies - Umberto Eco's Foucault's Pendulum, or alternately any episode of Scooby Doo.
The prose of the first couple of chapters is thunderously bad, but after that it settles into fairly routine rubbish. Some of the plot twists, like the first anagrams, the first password, the mysterious script, and the identity of the Teacher, seemed pretty blindingly obvious to me. None of it was particularly elegantly executed, and many historical details are simply wrong - as are some contemporary ones - Opus Dei members are not monks! In the book's defence, he doesn't overdo the mystical explanations, relying in the end on simple human explanations of his characters' far-fetched actions. And there is a certain charm in that most of the book takes place within a single twelve-hour period. But I really don't understand what people see in it, or why it has been so popular.