The writing and illustrations are great work, don't judge this book by the boring cover. If you want to play grimdark, it's great. Lots of ideas to incorporate into your own game, both for Fate and DungeonWorld players.
If you want to play grimdark in DW it's probably very useful, but as I mostly in the Fate camp I would say this is really underrated as a Fate supplement.
Rather than a hack of Fate for conversion to more DnDish stats, which you can find a lot of here on DriveThruRPG, this is stays true to Fate. Rather than a how-to of how to fit DnD stats into your Fate game, this book shows how to put GrimDark into your character. Playbooks for Fate is arguably a DW hack of Fate instead of DnD hack, but it's way more useful. What kind of character to play, and how, is the core of the setting in role playing games. The Fate playbooks are open-ended of course, but with lots of ideas for stunts and aspects it shows how to flesh out the character you want. The stunt-based magic is also well done, seemingly balanced and easy to incorporate in a game without lots of new mechanics.