If you were a wizard, you might say that this little blue book was the equivalent of Chairman Mao's Little Red Book - it's full of instructive tales for Hogwartians, a magician's manifesto 'translated from the original runes by Hermione Granger'. I enjoyed the unlikely exercise of reading it as if I really were a trainee witch. But the book was, originally, handwritten on vellum in a limited edition for a charity auction - and it still reads that way. It's a curiosity, a collector's item, an intricate in-joke complete with footnotes that will engross any close student of Harry Potter. It is satisfactory that the joke now costs a mere £6.99 (Amazon paid a record-breaking £1.95m for its limited edition copy at Sothebys last year). And it will make a good Christmas present, not least because all JK Rowling's royalties are going to the Children's High Level Group, the charity she co-founded to improve life for vulnerable children.

The book contains five fairy tales written, ostensibly, in the 15th century by Beedle the Bard (possessor of an 'exceptionally luxuriant beard'), and each one is followed by windy academic commentary from one Professor Dumbledore. Rowling is herself responsible for the illustrations which are, in their spindly way, charming. But what is most likeable about the book is everything it fails to teach the reader. Rowling has a keen - and refreshing - interest in the fallibility of magic, and she celebrates ordinary, human acts of faith too, as an alternative to sorcery. 'The Fountain of Fair Fortune' performs a miracle for a knight known as Luckless - although actually, it emerges, the fountain does not flow with an elixir but with ordinary H2O. Luckless makes his own luck by believing in it: a fairy-tale placebo effect. And the book also offers a tiny but revealing hint of what life might be like for JK Rowling as she convincingly describes, in the final tale, the uses to which one might put an 'invisibility cloak.'

JK Rowling inserted a kind of fairy tale into Harry Potter and the Deathly Hallows, the seventh and last Harry Potter tome. In his will, Professor Dumbledore leaves Hermione Granger his copy of The Tales of Beedle the Bard, a collection of children's stories, and she later reads one out. In "The Tale of the Three Brothers", the brothers meet Death and win from him an Elder Wand, a Resurrection Stone and a Cloak of Invisibility. The greedy brothers who win the Wand and the Stone perish by them; the humble brother with the cloak lives a long life protected by it from Death, until in old age he voluntarily relinquishes it. These magical objects are the "Deathly Hallows" of the book's title. The story concerns the dangerous desire to vanquish death, a preoccupation in the book.

Now Rowling has given us Dumbledore's collection, adorned with her own drawings and sold in aid of the children's charity that she set up with politician Emma Nicholson. It is a thin volume, with just four more brief tales added to the reprinted "Three Brothers", and bulked out with Dumbledore's "notes". Rowling is beyond needing to manufacture spin-offs, and the collection probably did begin, as she says, as a jeu d'esprit. Yet the fairy story is a tricky form, and it is not clear that Rowling's inventiveness and humour are suited to the genre.

One problem is magic. Some of the most haunting stories of the Grimms or Hans Christian Andersen get their force from the eruption of the supernatural into the ordinary world. The ending to a fairy tale characteristically involves the end of magic: a curse is lifted, a spell is broken. Rowling explains that in her tales the heroes and heroines are familiar with magic already. In the first tale, "The Wizard and the Hopping Pot", a young wizard inherits a magic cooking pot from his father and has to learn to use it for the good of others. In a traditional tale, the son would be a selfish boy who was being admonished from another world. In Rowling's version, he is more like a geek having trouble with a new machine.

A second story, "The Fountain of Fair Fortune", features a twist that many will recognise from "The Emperor's New Clothes": its characters believe their lives are being transformed by magic when in fact the fountain that brings happiness to each of them "carried no enchantment at all". Yet here too the spin is taken off the narrative by characters who are themselves capable of magic. A love-lorn witch uses her wand to draw from her mind all memories of the lover who has deserted her: a nice psychotherapeutic trick if you can do it.

The Potter books allow for endless discussion of the rules by which magic works. In a fairy tale, in contrast, supernatural logic must be self-defining. So in "The Warlock's Hairy Heart", the story of a misogynistic wizard, it is frustrating that the protagonist can magically remove his own heart in the first part of the story, yet fatally cannot at the end. This story, like "Three Brothers" and "Babbitty Rabbitty and her Cackling Stump", contains violent death or the threat of it, and Rowling's illustrations mingle flowers with memento mori emblems. She certainly does not shirk the grimness that sometimes defines the genre.

The humour is in Dumbledore's notes. There is a certain amount of mock-scholarly fun, and the opportunity for the author to get back at some of her more pious critics. But for all their air of self-mockery, the notes protest too much on the part of Rowling's own endeavour. When we hear that a tale has, in the good prof's opinion, survived down the centuries because "it speaks to the dark depths in all of us", or that "this story made a profound impression on me as a boy", you feel that he is making claims for the power of not very powerful stories.

Tales such as these, Xenophilius Lovegood informs Hermione in Harry Potter and the Deathly Hallows, are "told to amuse rather than instruct", and many famous fairy stories are notably amoral. In Andersen's "Little Claus and Big Claus", the poor farmer wins peace and contentment by tricking his rival into being sewn into a sack and thrown into a river. The soldier in "The Tinderbox" cuts off the witch's head when she refuses to tell him why she wants the tinderbox, yet he goes on to win the hand of the beautiful princess. Throats are cut and hearts removed from bodies in Rowling's tales, but they manage nothing as blithely cruel as the best fairy tales.

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