Illuminatus! Trilogy by Robert Anton Wilson. I've reread it at least five times now, and it's so dense with historical and literary references and parodies interweaved into its plot that I notice new things every time I reread it that I previously thought were things the author made up, were actually subtle (or not so subtle) references.

Plus the plot bounces around constantly, sometimes mid sentence, between characters and times and realities, that you get a better feel for the plot when you reread it anyway.