[DeepL Translation - needs review] The war between exorcists and Akuma was brought to a virtual end of the 19th century. During the war, a file was created by the military organization "Black Order". The official data of the Exorcists and the forces of the Millennium Counts, etc., recorded in the file are now revealed!
(Translator: DeepL)
DGM's first fanbook
灰色ノ聖櫃 is D.Gray-man's first fanbook, published just after volume 15's release. (Note that there are some images/information from volume 16 in here, though they aren't explicitly marked.) It offers a huge amount of information, which I'll list below, but before that, I'll mention that while the text itself in terms of vocabulary or grammar isn't particularly difficult (if you've been reading DGM up until this point you probably won't have too many issues), any difficulty is going to come from there being a lot of it. Text is crammed onto every possible bit of white space, and there are illustrations all over the place, both beside and behind the text, which can add to the visual congestion. There is full furigana, and most of it is readable, if quite tiny.
Now, onto what the fanbook covers:
So as you can see, quite a bit packed in there. There won't necessarily be anything new here for serious fans of DGM; maybe a nugget of "oh I didn't know that" scattered here and there. I appreciated having ZONE printed here (it's the only published non-magazine work I'm aware of right now where it's easily obtainable), and the author interview was a nice glimpse into what the author's time pre-DGM looked like.