This is just such a weirdly-conceived book.
I think it wants to be a more cerebral Da Vinci Code, but what it ends up being is neither particularly cerebral nor particularly successful as a thriller.
It follows Edward, a hotshot financial lawyer person (I think?), as he's hired by the mysterious aristocratic Went family to catalogue their extensive library. Soon, the Duchess is asking him to find a codex (or, in normal English, a book) in the library which holds the key to some mysterious and terrible secret about the Went family.
There are a few interesting bits - I particularly found medievalist Margaret's speculation about the codex's contents (she thinks it's a fake) fairly convincing. But some of Grossman's history is just plain wrong (Chaucer was the only person reading Dante? Um, not so much, given that Dante was the thirteenth-century equivalent of J.K. Rowling), and the novel is strangely and aimlessly plotted, with Edward spending much of his time playing a computer game that turns out to have only a tangential relationship to anything.
The secret the codex contains is disappointingly mundane, and the ending of the book just seems to continue the theme of overall aimlessness. I don't know why we're supposed to care here.
(Oh, also? Edward's attempts to contact Margaret after their first chance meeting in a library are seriously stalkerish.)