Had a lovely time reading this, and the idea and presentation is superb!
I loved the idea of ‘epistolary deduction’ as a genre, and I don’t see many other games attempting this particular niche. The document presents what could have easily sounded like a dry subject in a clear and appealing way. I could get a good sense of the potential gameplay as well as the overall progression.
The length and detail of the document is pretty perfect, it reads easily while also being comprehensive and concise. I especially like the attention given to art and narrative, since these are two areas which really sell the idea of the game. Perhaps the only detail I can criticize would be the lack of a diagram to describe the overall interface and how a potential letter or puzzle might look - I found myself drawing a blank at times when imagining exactly how it would be laid out on a screen.
The way the art aesthetic is described is mouth-watering and I’d be prepared to check out the game based on that alone. The narrative was also really well-presented and would definitely hook me - especially the contrasting backgrounds and personalities of the characters. The deductive-style gameplay could have used a little more detail about how difficulty would be scaled, and how the epistolary puzzles would be kept interesting until the end with variations/twists (using the Strix Grimoire for example?) - but free-deduction games are some of my favorites and I’d definitely want to give this one a try.
Overall it’s a top-class GDD and I’d be one of your first wishlisters if this ever gets made :)