By Christine Kearney

NEW YORK (Reuters) - Billionaire Harry Potter author J.K. Rowling would feel "exploited" if a fan's unofficial encyclopedic companion to the boy wizard series was published, she said in court papers made public on Thursday.

Steve Vander Ark has written "The Harry Potter Lexicon" -- a 400-page reference book based on his popular fan Web site (www.hp-lexicon.org). Rowling and Warner Bros. are suing RDR Books, which planned to publish the book last November.

"I am very frustrated that a former fan has tried to co-opt my work for financial gain," Rowling, 42, who wrote the seven hugely successful Harry Potter novels, said in a declaration filed in U.S. District Court this week.

"I believe that RDR's book constitutes a Harry Potter 'rip off' of the type I have spent years trying to prevent, and that both I, as the creator of this world, and fans of Harry Potter, would be exploited by its publication," she said.

The lawsuit filed in October names RDR Books, an independent publisher based in Michigan, and unidentified persons as defendants. It seeks to stop publication and requests damages for copyright and federal trademark infringement and any profits to be gained.

Rowling has said she plans to write her own definitive Harry Potter encyclopedia, which would include material that did not make it into the novels, and donate the proceeds to charity. The novels have sold more than 400 million copies.

"I feel intensely protective, firstly, of the literary world I spent so long creating, and secondly, of the fans who bought my books in such large numbers," said the British writer ranked by Forbes as the world's 48th most powerful celebrity.

RDR Books has said Vander Ark, a librarian, had spoken at Harry Potter academic conferences in Britain, Canada and the United States and that a timeline he created was used by Warner Bros. in DVD releases of the Harry Potter films.

The company and Vander Ark have said the book would only promote the sale of Rowling's work and that Vander Ark's Web site, used by 25 million visitors, had been called "a great site" by Rowling herself.

Warner Bros is a unit of Time Warner Inc, which owns the copyright and trademark rights to the Harry Potter books.