Readers must cop it when a good book ends, grieving for love lost. Authors must wander around directionless.
In Rowling's case, she has solved the problem not by providing a sequel, but an elaboration of part of the final Potter volume, Harry Potter and the Deathly Hallows.
The Tales of Beedle the Bard delves into an element of the final Potter book in which the adolescent wizard vanquishes the evil Lord Voldemort. One of the stories, The Tale of The Three Brothers, contains clues that helped Harry in his quest.
In this slim but handsome, just-in-time-for-Christmas volume, all five tales are translated from Beedle's original manuscripts by Potter's school friend and conscience, Hermione Granger.
The Tales of Beedle the Bard also come with notes on their underlying meaning by the headmaster of Hogwart's School, Professor Dumbledore, who left them to Granger. And there are footnotes on Dumbledore's notes by Rowling herself.
This blurs the distinction between author and subject; between fantasy and reality. Like all good reads, it is a meld that allows the reader to relate to the experiences of the literally fantastic subject.
Rowling hand-wrote and illustrated just seven copies of Beedle after finishing the Potter series proper, giving six copies to people who had helped her make Potter a success and selling the seventh at auction for £1.95 million ($A4.44 million) to online retailer Amazon.
But now she has expanded the print-run to help the Children's High Level Group, a charity which Rowling co-founded in 2005 to support institutionalised kids.
The Beedle stories purport to be fairy tales for child wizards and witches, just as conventional kids - muggles, in the Potter parlance - learn about life, about right and wrong, good and bad, strong and weak, virtue and sin, from fairy tales such as Goldilocks and the Three Bears.
Indeed the fourth of the Beedle tales, Babbity Rabbity and her Cackling Stump, is a loose parody of that classic fairy tale of folly and self-delusion, The Emperor's New Clothes.
Beedle is more original than that, though. Rowling grew as a story-teller as Potter and his school friends grew as individuals through the seven Potter volumes. They started out as Year 7 newbies, but ended up, as it were, matriculating. We grew with them.
In the end, they weren't books for kids, but for adolescents and grown-ups. Rowling's language is down to earth, but mature and subtle and, for that, richer than children generally might appreciate.
Rowling extends the Harry Potter story for us through Beedle by making it deeper - but in a lighter way that helps us ease ourselves off the Potter hook.
But as the seven volumes of Potter's school years grew longer as the protagonist grew older, Beedle reverses this trend with just 128 pages of large, well-spaced, elegant type.
This doesn't make the book more simplistic or necessarily accessible to young children.
Rather, it summarises the moral lessons of the seven volumes, as if Rowling is wrapping up the saga and tying up the lessons for us grown-ups to cogitate on soporifically in our armchairs on Christmas afternoon.
Maybe now she, and we, should move on.
