As soon as I heard this book was coming out, I knew I had to have it, and I'm delighted to report that my faith was not misplaced. I've spent the last week reading it cover to cover, and have gleefully soaked up every word and image.
This isn't just an art book or game guide. This is an in-world encyclopedia that basically takes all of those codex entries you pick up during the course of game-play and organizes them around breathtaking concept art. There's a lot of new information as well, and an incredibly helpful timeline that spans the course of the book to help you put world events in order. They don't go into a /lot/ of detail about any one thing, but you get a good sampling of much of what makes up Thedas (the nations, the historical events, religions, races, the Blights, etc.). I imagine future volumes will be more focused, but this is just my prediction/hope.
Admittedly, I don't think this book is for someone who only has/had a passing interest in the games. I think this book is for the players who can't keep themselves from revisiting Thedas time and time again (I myself just finished my fifth play-through of Origins, have a healthy start on my fourth play-through of Awakening, and an itch to go through Dragon Age II again ASAP). If you didn't just like Dragon Age, but /loved/ it, this is definitely the book for you.
I can't wait to see what future volumes hold!