The study of invisible writings was a new discipline made available by the discovery of the bi-dimensional nature of Library-Space. The thaumic mathematics are complex, but boil down to the fact that all books, everywhere, affect all other books. This is obvious: books inspire other books written in the future, and cite books written in the past. But the General Theory of L-Space suggest that, in that case, the contents of books as yet unwritten can be deduced from books now in existence.

Terry Pratchett, Lords and Ladies