It is being maintained by Dwellers and enlarged by Dwellers. They also store loads of (un)known tech here.
Inside the Library there are holo-books wich contain holographic pictures and Dweller audio tracks wich explain the history of a nation or tech.
Holo-Disks are small(size of a bolded hand)objects wich(sometimes)store thousand of years of history of a single nation. But it is more commonly used to store a detailed blueprint of a single piece of tech in them.