First mentioned in Deathly Hallows, Rowling made the work of fiction in fiction into a real book. Just a bunch of children's fairy tales but for the magical kids of the Wizarding World.
I didn't really like the hopping pot one.
I did like the fountain one. Very good tale.
The Warlock's Hairy Heart one was easily the worst.
I do like the one with the foolish king and the fake wizard who get tricked by a real witch.
And of course we have the tale of the 3 brothers. Which you can already read (or see in the film) in Harry Potter and the Deathly Hallows.