The rule of four - Ian Caldwell

Tom Sullivan, about to graduate from Princeton, is haunted by the violent death of his father, an academic who devoted his life to studying one of the rarest, most complex and most valuable books in the world. Coded in seven languages, the Hypnerotomachia Poliphili, an intricate mathematical mystery and a tale of love and arcane brutality, has baffled scholars since 1499.

Tom's friend Paul is similarly obsessed and when a long-lost diary surfaces, they finally seem to make a breakthrough. But only hours later, a fellow researcher is murdered and the two friends suddenly find themselves in great danger. Working desperately to expose the book's secret, they slowly uncover a Renaissance tale of passion and blood, a hidden crypt and a secret worth dying to protect.