Bidder pays £28,000 for Harry Potter's secrets ...

A card containing a jumble of words providing an insight into the secret plot of the long-awaited new Harry Potter book was sold at auction for £28,680 yesterday.

The 93-word teaser, hand-written by the Harry Potter author, J K Rowling, was bought for almost six times its £5,000 reserve price by an American collector who wished to remain anonymous.

The story of Harry Potter and the Order of the Phoenix is a closely guarded secret, with Sotheby's revealing only that the words "Ron ... broom ... sacked ... house-elf ... new ... teacher ... dies ... sorry" are in the teaser. Written in blue ink, the lot was sold "sight unseen" – meaning a buyer could choose to keep the plot a secret.

J K Rowling produced the signed card at a Society of Authors conference earlier this year when writers were asked to submit items for auction to help benefit Book Aid International. The charity said the £24,000 profit from the Rowling sale would buy 18,500 books for schools in African countries.

Sara Harrity, Book Aid International director, said: "It's a wonderful Christmas present for readers in some of the world's poorest countries."

A Harry Potter website representing fans worldwide was quickly defeated in its £15,000 bid for a sneak peek at the novel but the group, www.the-leaky-cauldron.org, donated the sum to the charity nevertheless. Melissa Anelli, managing editor of the site, said: "We were worried that a wealthy fan would win it and keep its contents private from the millions of fans eagerly waiting for more than two and a half years to find out about the next book. We can only hope that the person who got it shares it."

Meanwhile, a hard-backed version of a Harry Potter book bought by a woman from Swindon for £10.99 five years ago was sold at auction yesterday for £13,000. Dominic Winter Book Auction in Swindon, which sold the book, said it was one of just 300 printed. Only a few are privately owned. It was bought by a book dealer from Bristol.

The seller, Monica Timms, 56, said: "I just need to sit down and get over it now."