I just tried to go on tester I have full access to final boss so it seems like a part of library that hasn't been discover yet.
My theory is that door is opened by the other side and you actually have to open this
https://imgur.com/a/X60h6UG the wall is to close to that seems like it could be lifted or something to give access to that area seems like there are no monsters on the other side of it.
There is also this in front of it
https://imgur.com/a/gpJ1nyO the floor has something written but I cant read it also the eyes on the wall the move to the left middle and right except for the one in the middle is always looking straight.
Those eyes are the same that in other part of library they throw a line of fire, in the entrance the statues that are throwing holy are also in the floor that the sealed door is and between them there is the alphabet and numbers 0-9 but there is one statue that doesn't appear in the entrance the statue of a cyclops slave I think it was the name.
Find out more things like there are 4 ¿levers? There are 4 things in shape of hands with a book in the middle 3 on the north and one on the left of library also on the south are 2 room with mechanism like a stepping floor. Sadly I don't have the people interest to keep my investigation the levers keep up for 10 seconds when you activate them and wiki says it will change something depending on the boss room you are. This time I couldn't take pictures but I was streaming so I will get the images as soon as I can. Thanks to whoever fixed my multiple comment in just one I'm still learning about TibiaAa and were posting things in the other I was founding them. There are also a lot of books which content can be discussed and a one is very interesting seems to be a number code.
Also, I almost forgot there is a corridor the same you get to the sealed door. There is an alphabet and at the end the numbers 0-9 but some of the walls have a little purple smoke and lightning the order is.
Smoke
A E I N P X
Lightning
B G M V Z
if you look only to the words no matter if it smoke or lightning is something like this
ABEGIMNPVXZ
New update
I have found 26 books around all library. I cant say it's all of them because library is a big place and hard to explore due the danger and since I did it in the test and its coming to an end. I wrote the information in a note block and upload it to mega so anyone can have it in case they want to think about them without having to risk going there
https://mega.nz/file/B4YWmAIC#rjiRI_rGYilSM2s6RKE7M6M6OpFdmfBk6tjPlvLXjMw