A smiling J K Rowling arrived at a Manhattan court today to testify against the publisher of a Harry Potter encyclopaedia.

Ms Rowling brought the copyright lawsuit against RDR Books, the publisher of the "Harry Potter Lexicon" - written by 50-year-old school librarian Steven Vander Ark.

Wearing a grey pinstriped jacket and knee-length skirt, the writer did not speak as she entered the courthouse where she will testify this afternoon.

The trial comes eight months after Ms Rowling published her seventh and final book in the Harry Potter series. They have sold more than 400 million copies and been translated into 64 languages, as well as spawning a film franchise that has generated $4.5 billion ($2.8 billion) at the box office.

Ms Rowling said she enjoyed Mr Vander Ark’s Harry Potter Lexicon website but argued that the book, which has a price of $24.95 ($15.75), failed to include any online commentary and was “nothing more than a rearrangement of her own material”.

Dan Shallman, one of the writer’s lawyers, told Judge Robert P Patterson – who is hearing the trial without a jury – that Rowling “feels like her words were stolen”. He said the author felt so personally violated that she went through her books identifying alleged similarities with the lexicon.

David Saul Hammer, RDR's lawyer, said the publisher will not dispute that much of the material in the lexicon infringed her copyrights.

But he will argue that it was used for a scholarly pursuit and was therefore legal.

In court papers filed prior to the trial, Ms Rowling said she was “deeply troubled" by the book.

“If RDR’s position is accepted, it will undoubtedly have a significant, negative impact on the freedoms enjoyed by genuine fans on the Internet,” she said. “Authors everywhere will be forced to protect their creations much more rigorously, which could mean denying well-meaning fans permission to pursue legitimate creative activities.”

Mr Vander Ark was a teacher and school librarian in Byron Center, Michigan, before recently moving to London to begin a career as a writer.

He said he joined an adult online discussion group devoted to the Harry Potter books in 1999 before launching his own website as a hobby a year later. Since then, neither Ms Rowling nor her publisher had ever complained about anything on it, he said.

In May 2004, Ms Rowling allegedly mentioned his website directly, writing: “This is such a great site that I have been known to sneak into an Internet cafe while out writing and check a fact rather than go into a bookshop and buy a copy of Harry Potter (which is embarrassing). A website for the dangerously obsessive; my natural home.”

The website attracts about 1.5 million page views per month as well as contributions from people all over the world, Mr Vander Ark said.

He said he initially declined proposals to convert it into an encyclopedia, in part because he believed until last August that doing so would breach copyright.

But Mr Vander Ark was told by an RDR employee that publishing the lexicon would be legal, he said.

The author insisted that RDR included a clause in his contract that the publisher would defend and pay any damages that might result from claims against him.

He said it was decided that the lexicon would include sections from the Lexicon Web site that give descriptions and commentary on individual names, places, spells and creatures from Harry Potter stories.

In his court statement, Mr Vander Ark said the lexicon “enhances the pleasure of readers of the Potter novels, and deepens their appreciation of Ms Rowling’s achievement.”

On Friday, Mr Hammer said Ms Rowling’s lawyers did not want Vander Ark in the courtroom while Rowling testifies.

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