The narrative in your GDD is fantastic, and the clarity of the doc is amazing as well. Your information was quite to the point, and I had little difficulty understanding the mechanics and how the game would function. Really the only critique I had was the inclusion of the Grimoire- will the documents be kept in there, or is it simply another document? I think having some sort of puzzle tied into it would help enhance the magic themes of the game, and would provide an additional avenue for puzzles and an addiction place to cross-reference clues from. If not, I'm not too sure of its role in the game, aside from being a part of the lore.
Or I read it wrong! Overall, very nice GDD, great narrative, and I would certainly play it.