Ubisoft will release Assassin's Creed: The Encyclopedia as part of the Assassin's Creed: Revelations "Animus Edition", a super fancy, not very cheap limited edition for European gamers. That encyclopaedia features some of the following artwork, assassin's from all walks of life and stages of history.
US gamers won't get the pricey "Animus Edition" with its fancy encyclopedia, but they can order it separately from Ubisoft's online store. After seeing some of the Assassin's Creed themed artwork featured within - and some of the pieces submitted, but not confirmed to be in the encyclopaedia - it's starting to look like a good purchase regardless of how one gets it.
What follows is Assassin's Creed: The Encyclopedia art and submissions from artists - like Grzegorz Domaradzki, TomX/SeedSeven, Will Murai, JamesNG, David Alvarez and others - who may or may not be included in the final product.