There was actually going to be a note as you said but I just haven't had enough time to create system where you can read them. It is a bit too hard without it. I feel the note was really important and i should do it sooner. Here's the solution - the key is in separate room that you can enter through the door on the right. You need to open them first. There is a special book that acts as a lever. It's a green one.
Thanks for your time and the feedback. I really appriceate it!
And sorry for the time you wasted throwing the books around. I'm planning to expand this game and fix all of these issues. If you are intrested in it give me a follow on itch.io. I'll do my best to not dissapoint you.