definitely Foucaults Pendulum. it's like the DaVinci Code but for medieval history buffs plus the plot is wickedly clever
The same could be said of Dostoyevsky. Those long, drawn out passages are often there to make a psychological impression upon the reader rather than adding anything to the plot line per se.
Though at times it did read like a database dump (albeit a fascinating one), the intricacy of the background of theories and history is all in service to the ridiculousness of the plot. Without it, the profound stupidity of the formulation of the meta-conspiracy would not have been believable.