Harry Potter author JK Rowling said she would feel "exploited" if an unofficial reference book about the boy wizard was published, according to court papers.
JK Rowling published her final Harry Potter novel last July
The writer is taking legal action in New York against Steve Vander Ark and publisher RDR Books over their plans to release a Harry Potter encyclopaedia.
"I am very frustrated that a former fan has tried to co-opt my work for financial gain," Ms Rowling said.
Lawyers for RDR Books said it was a "legitimate literary activity".
They added: "Like a reference book or a guide to literature, it's a long-recognised genre.
"We are not replacing the novel or taking away the market."
Ms Rowling has said she will write her own definitive Harry Potter encyclopaedia, including material that did not make it into the novels.
"I believe that RDR's book constitutes a Harry Potter rip-off of the type I have spent years trying to prevent," Ms Rowling said in a written declaration to the court.
"Both I, as the creator of this world, and fans of Harry Potter would be exploited by its publication.
"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."
The contested book, Harry Potter Lexicon, is based on material from an internet fan site of the same name.
On its website, RDR Books said it was "determined to publish this book for the benefit of Harry Potter fans everywhere" .
The publisher agreed to delay the release of the book after Ms Rowling sought an injunction against its release in October.
It had been scheduled for publication on 28 November 2007.
The case against the publisher and the writer is being brought by Ms Rowling and Harry Potter copyright-holders Warner Bros.
Ms Rowling's Harry Potter novels have sold more than 400 million copies worldwide.