L-space is a concept described by Terry Pratchett.
It is well-known to all visitors of small second-hand book shops that the shop seems to hold much more books than what seems possible from the outside of the shop. This is due to the fact that large masses of books actually bend space. All libraries and book-shops are connected through the so-called L-space, which is some kind of twisted space between the books, where it is possible to travel limitless distances. It is a bit hard to describe, but it is perfectly understandable if you've encountered a lot of books at one time...
Not only book-collections of our own world are interconnected. Any library of any world at any time can be reached if you know exactly what you are doing. That is the reason why many keepers of the aforementioned second-hand book-shops seem a little alien. They are alien (though usually very nice). And in their home worlds, it is perfectly normal to go around in slippers all the time and keep opening hours from 7pm to midnight.
Paraphrased from Guards! Guards! by Terry Pratchett

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