What is the Interstitial Library?

The Interstitial Library is that infinite collection of books?obscure, lost, or not yet written?that belong in the spaces between the books on a library shelf.

Where is the Interstitial Library? Can I visit it?
The Interstitial Library is located in the Interstitial States?labyrinthine passages outside ordinary space-time. Visitors are welcome.

How do I get to it?

Generally, by sliding between two books on your bookshelf, or by correctly pronouncing the word “hh.”