There are indeed books in that library; you just won’t ever be able to go get them yourself. The Mansueto Library was designed around a new robotic storage system. The main reading room of the library is built on a 5 story deep book vault. The vault contains a total of 3.5 million books, which is comparable to major universities. The books are packed tightly in drawers, and only the system knows where everything is.
When you request a book, the librarians will send the robotic crane to retrieve the appropriate drawer. The drawer will be brought up to the reading room where the librarian will remove it, check it out, and send the drawer back down to the catacombs.
You can watch the following video to see how this library functions.
via Singularity Hub