First off, Barlowe is an amazingly talented artist, and anything he does is worth owning.
Second, this book is an excellent continuation of "Inferno". If you do not own "Inferno", this can be read alone. However, the two are part of the same vision of Hell.
Third, while this book has slightly less content than "Inferno", it's still excellent. The artwork is inspired and haunting. The demons have an organic feel that makes them look real. They also have the remnants of their angelic heritage. Whereas "Inferno" is Barlowe's travels through Hell, focusing on people, places, and "beings", this book focuses on beings in the hierarchy of Hell, from officers to demons to fallen souls.