This is a fairly uproarious comic novel about the fine line between truth and fiction, that also functions as a critique of medieval logic and reasoning and as a celebration/satire of the power of myth (and faith, and belief). But I felt a nagging sense of deja vu the entire time I was reading it.
Because, though the story is drastically different than Foulcault's Pendulum in terms of setting, characters and their goals, and the target of the critique - in this case the kind of backwards reasoning and reliance on belief over fact that gave us the ontological "argument" for the existence of God from St. Anselm, for example, of whom Eco himself once said "Saint Anselm's ontological argument is moronic...God must exist because I can conceive Him as a being perfect in all ways, including existence. The saint confuses existence in thought with existence in reality." - and though, from memory, this is a much funnier book than Pendulum, it still seems like he's making the same point and the general conceit seems the same. Namely, when people allow belief to determine or cloud their reasoning, they will eventually believe anything.
It feels like it has the exact same reason for existence as Eco's greatest novel. And so I say to you that you should read Foucault's Pendulum, the best satire of conspiracy theory I have ever read, than this novel, which is more of a satire of medieval imagination substituting for reason and argument, and which just feels like a spiritual sequel to - really, a retread of - Pendulum.