I've just passed the halfway point in
Foucault's Pendulum and am enjoying it immensely. Also slightly kicking myself for not delving into it years ago, but never mind. Criticisms are that the female characters seem a bit unbelievable and exist mainly as romantic/erotic foils to the male characters who are actually concerned with the central plot, and some of the verbal flights of fancy go on a bit, but these are minor quibbles really.
Other good Conspiracies In Literature: the various Canadian/Québécois separatist groups - chief among them, the notorious and dreaded
Assassins des Fauteuils Rollents! - in
Infinite Jest. All the bits involving them were among my favourite parts in the book.
Edit: haha, better late than never, eh.
Foucault's Pendulum by Umberto Eco is kinda in the right area.
Mr. Tea in 2007 said:
I've heard good things about that book, I suppose it's on my eventual to-read list.