I read Foucault's Pendulum and finished it, but it was heavy going. I then read "The Illuminatus! Trilogy" [0] which I enjoyed much more, because it was more intentionally cranky and funny. It is to conspiracy novels as "Airplane!" was to disaster movies, and then some.
[Edit] Both of them make Dan Brown look really, really unimpressive. If "The Da Vinci Code" is a vaguely thought-through concept aimed at general readers, "The Illuminatus! Trilogy" is an explosion in a conspiracy theory factory.