The way that books used to be printed, the reader would have to cut open each page with a paper knife before it could be read, every page a tiny gift from the writer.
The printing happened on large sheets of paper which were then folded into rectangles the size of the finished pages and bound. The reader then sliced open the folds. Paper knives, variants of letter openers, were used for this purpose.
The deckle edge on modern books is an imitation of what those sliced open books looked like.