AT £308 per word, it is one of the most expensive postcards in history.

A small card, measuring 150mm x 210mm and containing a 93-word excerpt of J. K. Rowling’s latest Harry Potter book, was sold to an American collector for £28,680 yesterday.

The full contents of the card, including the words “Ron . . . . broom . . . sacked . . . house-elf . . . new . . . teacher . . . dies . . . sorry”, is a closely guarded secret and will probably not be revealed until Harry Potter and the Order of the Phoenix is published, probably next year.

It is the first “teaser” anyone, including Ms Rowling’s publishers, has seen of the fifth volume in the seven-book series. The buyer, who has asked to remain anonymous, will not be able to reproduce the words without permission from Ms Rowling because she retains the copyright.

Neil Blair, from J K Rowling’s literary agents Christopher Little, said that the rationale for the appearance of the card at Sotheby’s literature sale yesterday was not for the words to be published, but to raise money for the charity Book Aid International.

Ms Rowling’s agents say that the author is putting “final tweaks” to the plot. Mr Blair added that the publishers had not seen anything of the next episode of Harry Potter, where the young wizard enters puberty — “unless the mysterious buyer was the publisher”.

Put in the context of the fees Ms Rowling’s publishers Bloomsbury and Warner Brothers have paid for the rights to the 500,000 words of her works so far, the word rate for the card is quite reasonable. According to estimates that suggest Ms Rowling is worth £226 million, each of her words is worth roughly £452.

That compares with £250 per word for a first edition of Beatrix Potter’s Tale of Peter Rabbit, which sold for £40,000 in July, 43p for every word in a £43,020 copy of The Hobbit, and 66p a word in a first edition of Winnie the Pooh, which fetched £100,150 yesterday.

Ms Rowling is not the writer of the most expensive postcard, however. The oldest known postcard, sent to the writer Theodore Hook in 1840, sold for £31,000, roughly £344 per word.

That makes the signed first edition of Harry Potter and the Philosopher’s Stone sold for £13,000 seem a snip at 17p per word.

Or of course, you could try a bookshop — and pick up all four volumes at a mere 0.0035p per word.

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