Bloomsbury Publishing, the home to Harry Potter, said it was already reaping the benefits of next month's publication of the sixth novel in the boy wizard series, but investors still marked down the shares in late morning trading.

Nigel Newton, the chairman of Bloomsbury, said sales of previous books in the Harry Potter series had increased in advance of the release of the penultimate novel by JK Rowling: Harry Potter and the Half Blood Prince. Mr Newton said preparations for the book launch were "progressing in line with our expectations".

In an effort to maximise sales of the hardback edition, the publisher will begin its biggest ever consumer advertising campaign on July 4, in advance of the official publication of the book on July 16.

JK Rowling, the now multi-millionaire creator of the adventures at the Hogwarts' School for wizards, will read from the novel in the Great Hall of Edinburgh Castle in front of an invited audience of children.

Since the first novel was published more than six years ago, the Harry Potter books have become a publishing phenomenon. Some 5 million copies of the fifth novel, Harry Potter and the Order of the Phoenix, were sold in 24 hours following the book's release. Amazon.com, the online shopping site, has already taken more than 1 million advance orders for the latest book.

But investors did not share readers' excitement this morning. After closing last night at 379p, the shares were sold down by 6p, or more than 1.5 per cent, in late morning trading to 376.5p. Last night's close valued Bloomsbury Publishing at £275.19 million.

In a statement to the annual meeting that sought to highlight Bloombury's international business, Mr Newton said the merger of Walker Publishing with Bloomsbury's business in the United States was "well under way". He said cost cuts and operational efficiences had been introduced.

"With the new infrastructure in place, the operation will raise its output of titles through increasing its title acquisition rate and the publication of its retained paperback rights beginning in 2006," Mr Newton said.

As well as highlighting its British publications - including a project by Ben Schott, who won success with his fact-filled book Original Miscellany - Bloomsbury pointed to its latest bestseller in Germany, Die Siebte Nacht, and the success of the German version of Schott's book, Sammelsurium.

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