[@Fubsy]

There are no consequences.
Getting there is enough proof that you are worthy of entering, for few know the secret way to enter.
The library has no guards, but there is also method of cataloging or searching for something.
Therefore the sheer amount of knowledge actually obscures knowledge.
You'd be looking for one line of text in a sea of words, found within a volume of pages kept within tome that is surrounded by thousands upon thousands upon thousands of them. 
If you can find what you are looking for, then just like finding the Plane in itself, you are worthy of knowing what you wanted.

The only book that would be forbidden to read from would be his artifact. Something he uses to bestow concepts and knowledge, as well as record the secrets of the universe.

I envision a mortal seeking out his library to find the secrets of becoming an immortal.
At which X may probably smile and warn Muorival a mortal wishes to escape her grasp.