Reader in Invisible Writings
This - Ponder Stibbons' first role on the Faculty - involved the large-scale use of Hex. The thinking machine in the High Energy Magic Building was tasked with casting large numbers of Search spells (such as Weezencake's Unreliable Algorithm) virtually simultaneously (impossible for human wizards), trawling through L-space looking for fragments of written material. This way Hex 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. It took a huge amount of time and effort, but it was a success, proving that Ponder was indeed a Wizard worth the meals on offer at Unseen University.
"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).
One of the first books to come through as a complete entity was How to Dynamically Manage People for Dynamic Results in a Caring Empowering Way in Quite a Short Time Dynamically. Fragments of this turned up so often that Ponder was forced to believe that either it was either going to be an amazingly popular book or - possibly - that it was actually different books written almost identically.
Invisible Writings in Wiltshire
The last Discworld novel, The Shepherd's Crown, has a postscript by Rob Wilkins looking back on Terry's life as an author. Rob notes that in the last few years of his life, Terry's literary output vastly increased in a determination to get as much as possible out there before the inevitable end. He describes orphaned glimpses of novels never to be finished, Terry Pratchett's own Invisible writings:
"We will not now know how the old folk of Twilight Canyons solve the mystery of a missing treasure and defeat the rise of a Dark Lord despite their failing memories, nor the secret of the crystal cave and the carnivorous plants in The Dark Incontinent, nor how Constable Feeney solves a whodunnit amongst the congenitally decent and honest goblins, nor how the second book about Maurice as a ship's cat might have turned out. And these are just a few of the ideas his office and family knew about."
If we add the ones we already know about, such as Scouting for Trolls and Raising Taxes, then there is a vast Pratchett literature which will forever be in potentio somewhere in the Library, inaccessible to the vast majority of us.
(Afterword to The Shepherd's Crown, p336}