Unseen Library

In the discworld books which Terry Pratchett writes there is an Unseen University, and that university has a library

People were stupid, sometimes. They thought the Library was a dangerous place because of all the magical books, which was true enough, but what made it really one of the most dangerous places there could ever be was the simple fact that it was a library.
Guards, Guards, p.166

The study of invisible writings was a new discipline made available by the discovery of the bi-directional nature of Library-Space. The thaumic mathematics are complex, but boil down to the fact that all books, everywhere, affect all other books. This is obvious: books inspire other books written in the future, and cite books written in the past. But the General Theory of L-Space suggests that, in that case, the contents of books as yet unwritten can be deduced from books now in existence.
Lords and Ladies, p.55

Knowledge = power = energy = matter = mass, and on that simple equation rests the whole of L-space. It is via L-space that all books are connected (quoting the ones before them, and influencing the ones that come after). But there is no time in L-space. Nor is there, strictly speaking, any space. Nevertheless, L-space is infinitely large and connects all libraries, everywhere and everywhen. It’s never further than the other side of the bookshelf, yet only the most senior and respected librarians know the way in.
The Globe: The Science of Discworld II, p.38

Thanks to Chris Jones for sourcing the quotes