I learned about these from an article by Georges Dumézil on the Druids, Celtic traditions, and writing. Grab a teddy bear and settle in, kids:
[From the Welsh story of Lludd and Llevelys:] The evil sorcerers known as the Corannieit tyrannized over the kingdom of Britain. So artful were they that no conversation could be held anywhere on the island, no matter how softly one tried to speak, that they would be unable to overhear, if only the wind blew past. So no one could rise up and resist them. The king of the island, Lludd, furious at this state of affairs, wanted to get the help of his brother Llevelys, the king of France, to overthrow the sorcerers; so he went to visit him in France and yet even there, there was no privacy. The two kings put their heads together to find another way to communicate in such a way that the wind would not intercept the words traveling from mouth to ear and carry them back to the Corannieit. So Llevelys had his artisans make a long copper horn, through which the two brothers could chat securely. But the copper tube created echoes, noise, and static, so that whatever was said into it was turned to ugly and disagreeable words or a meaning opposite to that intended. Suspecting foul play by the devil, Llevelys washed the copper horn with wine, scaring the devil away, and thereafter it worked reliably. Llevelys gave his brother a magic formula for exterminating the Corannieit, which he did.
[From the Tales of E. T. A. Hoffman:] Albertine had three suitors. They were invited to choose one of three chests, one of which contained her portrait (and thus the promise of her hand) and the other two of which were consolation prizes. Tusmann, the nerdy suitor, opened one chest and found in it only a blank parchment book. At first infuriated, he was invited to put the notebook in his pocket and think of any book he wished he had. “I’m thinking of the Tractatus politicus by Thomasius, which I foolishly threw into a pond when I was contemplating suicide.”
“All right, now pull the notebook out of your pocket and take a look.”
“It’s the Tractatus!”
“Now think of another book you wish you had read.”
“How about The Battle of Composition and Harmony, an allegory about the invention of opera?”
“Now look in your pocket!”
“Hooray! It’s The Battle!”
“So you see now, with this magical book you have inherited the most complete and magnificent library in the world, and you can carry it with you wherever you go. All you have to do is make a wish, and the book you desire will magically appear in your pocket.”
Forgetting instantly about Albertine and his disappointment, Tusmann dropped into an armchair in the corner of the room, buried his face in the little book, and was from then on the happiest man on earth.
(Georges Dumézil, “La tradition druidique et l’écriture: le vivant et le mort,” Cahiers pour un temps, 3 )