I'm fascinated by concept artwork and the evolution of characters, ideas and environments. Despite not owning a video game artbook... yet (Rodea: The Sky Soldier should be my first), I do appreciate that kind of stuff.
Even finding that kind of content featured in-game is cool, Crash Team Racing and Ratchet & Clank come to mind. I just so happened to have played 'Psychonauts' for the first time, and that feature was very much present. I really hope 'Genei Ibun Roku #FE' gets an artbook, some of those screenshots look drop-dead gorgeous!
I like them, they're really cool looking. But they're usually oversized coffee table books, and I don't think their $40-$50 cool.
It should definitely be from a game that you were already going to purchase, from a franchise that you care a lot about, while remaining a suitable size for your consumer. For example, the 'Super Mario Maker' artbook seems nice and compact.