Ok I'll give you two vague hints, because most players seem to struggle with this puzzle. But be aware, even with the hints this puzzle will be hard to crack.
Best regards Christian
--- Warning spoilers below ---
Pay close attention to the quote at the front of the letter, you are probably missing an item.
Inventory interactions are possible on different hotspots of the letter.
Hi,
- SPOILER -
are you on day 1 or 2? If you are stuck on day 1 you should take a very close at your desk in the bookstore, the letter puzzle doesn’t have to be solved to finish day 1. If you are already on day 2 you can use the razor blade on multiple hotspots of the letter and you can use the letter on another inventory item you might be missing. To get it try the quote at the front of the letter on the book database.
Best regards Christian
