Visually stunning and it is absolutely perfect for a game like this. It feels like I'm doing some Elders Scrolls quest for the thieves guild. I do however have to add that the motion blur is not something I'm a fan of and would like to have an option to disable it.
When I got to the library I was super excited and intrigued when I found the key hole. But I still needed a key so I started looking... and looking... thinking ok its probably behind some of these books. So I started throwing them all out. And as I did that I was also like, are you really going to make me go through all of these shelves and not reward me with the key. And it turns out that is exactly what happened. Spent ten more minutes trying to find it but it was nowhere... Perhaps it ended up under one of the book piles from clearing the shelves but that thought eventually made me give up. Sorry I couldn't finish it.
So as a final thought I'd recommend you try find a good balance between keeping the player intrigued and spoon feeding them with hints as they explore. Perhaps fewer books to grind through, or perhaps a note from a librarian by the fallen bookshelf "I choose green because it was my favorite color and a shelf that had the least sunlight to prevent a glimmer from catching someone's eye" (assuming the key is hidden in some green book somewhere dark). You essentially want it to be just a bit more effort to find the item than to just google the walkthrough and ruin the immersion.
Hope this helps. Great game and very impressive you was achieved in such a short time. Keep at it!