Warner Brothers Entertainment / Amazon.com

The prelude to Harry Potter's life at Hogwarts is, if not exactly sweet, then at least very short.

A prequel to the billion-dollar fantasy franchise, handwritten by J.K. Rowling on both sides of a piece of paper barely bigger than a postcard, is one of 13 similarly crafted works being auctioned off for charity June 10 at a London bookstore.

Waterstone's Booksellers Ltd. doled out pieces of A5 paper (measuring about 4 inches by 6 inches) to Rowling, playwright Tom Stoppard, 2007 Nobel Prize winner Doris Lessing, Canadian feminist writer Margaret Atwood, High Fidelity scribe Nick Hornby and eight other writers and artists, asking for stories, essays or any other form of artistic expression that could be squeezed into the limited space.

Rowling, who has vowed not to write any continuation of the Harry Potter saga, nevertheless said she had "fun" penning the prequel.

Proceeds from the What's Your Story? auction at Waterstone's flagship store in central London will go toward the writers' association English PEN and Dyslexia Action, a British charity—both of which have reason to dream big. The last piece of Rowling's "extracurricular" work to wind up on the auction block—a leather-bound collection of fairy tales—went for $4 million.