Square Enix’s Ultimania books as seen above are a series of guides for their own games. Aside from a comprehensive walkthrough, they also feature artwork and other never before seen development footage.
This is also the case with Final Fantasy XIII (Battle Ultimania, Scenario Ultimania). Below are some pictures showing various concepts, renders and even PS2 alpha footage with Yuna and Rikku from Final Fantasy X:
You can view some more screens here: Source