http://store.steampowered.com/app/606100/
It comes with a launch discount! You can now also get both the artbook and the game itself in the Necromancer Edition Bundle (which comes with a small discount by default):
http://store.steampowered.com/bundle/2931/
Within the artbook, you will find all of the finished art pieces for the game as well as rough sketches, design notes and maybe a few little secrets from SoulSet's 'backstage'!
A whole 90 pages (!) of goodness!
A Wallpaper Pack is included with the artbook as a bonus.
We hope you'll enjoy!