Without spoiling too much of the fun here are some pics of interior artwork from the pre-order bonus. It's a 8.5 by 11 inch hardcover book, with an embossed trim bordering the cover. This isn't as high quality as other artbooks included in collector's editions (think Skyrim's padded leather number); although for a freebie this is quite a nice piece of promotional material.
Beyond the usual concept artwork, there's commentary and interviews with the developers. The whole package comes together as a real behind-the-scenes look at the game's conception.
Hope you enjoyed the quick preview!
**Quick update: when I said 'from Atlus' I should have specified 'from French developer Cyanide, published by Atlus'. I can't speak for Cyanide's games, but their last attempt at Martin's works didn't go over so well.