The Artbook is a 32-page book that came with the European Limited Edition and the Australian game if you preordered it. It includes official artwork of some enemies, the first 10 Masters, the main characters and some other things, such as a map of Imperia. It also includes some information on these first 10 Towers and the main characters, as well as some writing about the production of the game.
I have taken scans of the pages in the book. They are rather large images and not very high-quality scans, I'm afraid.