The Secret Crusade

Niccolò's Polo's journal was a detailed study of the life of the Grand Master of the Assassin Order, Altaïr Ibn-La'Ahad, by the famed explorer Niccolò Polo.

Compiled during Niccolò's stay in Masyaf in August, 1257, the journal covered the Grand Master's life from 1176, and the death of his father, to the Mongol siege of Masyaf in 1257. The journal also acted as a sort of diary, giving details of the Polo brothers actions during their stay in the village.

The journal was intented to pass into the possession of Niccolò's son, Marco, though by 1511 it had somehow been passed on to Ezio Auditore da Firenze.

Source

 * Assassin's Creed: The Secret Crusade