From Discworld & Terry Pratchett Wiki
L-space, short for library-space, is the ultimate portrayal of Pratchett's concept that the written word has powerful magical properties on the Discworld, and that in large quantities all books warp space and time around them. The principle of L-space revolves around a seemingly logical equation; it is an extension of the 'Knowledge is Power':
Large quantities of magical and mundane books create portals into L-space that can be accessed using innate powers of librarianship that are taught by the Librarians of Time and Space to those deemed worthy across the multiverse. Because libraries with enough books to open a portal are often large and sprawling, those venturing into L-space may not necessarily know that they have arrived. The floor and ceiling of L-space follow the floor and ceiling of the library used to access it; the best example of this is that the central dome of Unseen University's library is "always overhead" (Guards! Guards! - all quotes in this article come from this book). In every direction and as far as the eye can see bookshelves stretch off, meaning the nature of any walls are unknown.
Alternatively, it can be said that L-space manifests in our world in those obscure, hidden bookstores that, logic and the laws of physics insist, cannot possibly be as large on the outside as they appear on the inside. Somehow, after scraping one's shoulders against the improbably small door, one finds one's self turning one unseen corner after another, seemingly going on forever into further and more obscure sections as yet unobserved by human eyes. The town of Hay-on-Wye, known for having more bookshops per square mile than anywhere else in the world, contains many examples of this, and may be a substantial access point to L-space. Essentially, all bookstores are potentially infinite in extent; gateways into literary hyperspace: "[a] good bookshop is just a genteel blackhole that knows how to read."
Because L-space links every library, it is possible to reach any one of these throughout space, time and the multiverse. This means that there are potentially other forms of data storage other than books as it represents every library anywhere. Additionally, one can read any book ever written, any book that will be written at some point and books that were planned for writing that were not, as well as any book that could possibly be written. The first Reader in Invisible Writings was Ponder Stibbons whose job it was to get Hex to trawl virtually through L-space (which involved a hauge amount of simultaneous spell-casting, beyond the ability of any human wizard) looking for fragments of these possible books.
In order to actually travel through time personally, to see libraries under different skies, one needs the secrets. As this is a form of interdimensional and time travel, there are strict limits on its use, and the Librarians of Time and Space - that is, those who have access to L-space - have developed three simple rules to ensure abuse is kept to a minimum:
- Books must be returned by the last date stamped
- Do not interfere with the nature of causality
Senior librarians are also taught how to deal with the dangers of navigating L-space, such as the "harmless kickstool crabs, large and heavy wandering thesauri, the .303 Bookworm and the dreaded cliches, which must be avoided at all costs." Adventurers may find markings and scribbled notes on the shelves to help them navigate.
The Librarian moves through L-space back in time to discover when the book on the summoning of noble dragons was stolen and to confirm that it was stolen by the Elucidated Brethren. During his journey he sees himself asleep at his desk and is tempted to communicate, but realises that this would be breaking the third rule and stops himself. He does however leave the library and follow the thief through the streets, demonstrating how L-space can be used for time travel outside of the library itself. It was shown in the events of The Science of Discworld II: the Globe, that L-space can also be used to travel to alternate worlds, such as Roundworld, as well as through time.
After Vorbis has ordered the soldiers and Brutha to burn down the Ephebian library, and the flames start to rise higher, there is a paragraph describing how the Librarian appears with a sack, and then describes how several scrolls appear in the Unseen University Library, which were thought to have been destroyed in the great fire.
The Librarian joins the Wizards in a Lancre adventure to stop naughty elves from ruining the wedding, and ending the lives, of the new royal family, which includes former witch Magrat Garlick. Within this we learn "the thaumic mathematics are complex, but boil down to the fact that all books, everywhere, affect all other books." From there the nature of bi-directionalism is revealed to demonstrate that any book ever to be written can be found in any book not yet written. In mathematical terms, as noted in The Science of Discworld, L-Space represents a form of phase space. This made possible the study of invisible writings (also based on a similar theory to do with the infinite nature of Pi, and the ways in which, if one was to transcribe alphabetical values to the numbers of Pi, one could hypothetically find the contents of every book ever written. For more information see Bloody Stupid Johnson and the New Pie, featured in Going Postal.
In fact, as the entry on the Ankh-Morpork Post Office tells us,
When mountains of undelivered mail were gathered in one place, all the unread words strove to be read. The words reached out into any mind available in the vicinity, writing themselves across one's inner vision and whispering to one. The letters also created illusions of the good old days – the counters, the chandeliers, the upstairs balconies, the staff, the customers, the postmen, the bustle; the lights, the noises, everything. The Post Office came to be considered as haunted.
This describes the twenty- or thirty-year-old accumulation of undelivered mail at the Post Office as a powerful focus for the distortion of time and space, which, with the nearby presence of the Sorting Engine, sets up a localised L-space field all of its own which is powerful enough to lure four postmasters, and almost a fifth, into a fatal illusion of the Post Office at the height of its glory. This is as it should be: these are unpublished writings, after all, and are not volumes of letters regularly published in book form? Professor Ladislav Pelc, the Prehumous Professor of Morbid Bibliomancy, has made a lifetime's study of this subject at Unseen University, and is firmly of the opinion that the dead-letter pile at the Ankh-Morpork Post Office has over the course of thirty years reached a critical mass and established an L-field all of its very own.