She says her copyrights are being violated by a fan who plans to publish a "Harry Potter" encyclopedia.
The showdown between Rowling and Steven Vander Ark is scheduled to last most of the week in U.S. District Court. She'll spend her breaks in the seclusion of a jury room -- away from any die-hard Potter fans.
Rowling brought the lawsuit last year against Vander Ark's publisher, RDR Books, to stop publication of the "Harry Potter Lexicon."
The trial comes eight months after Rowling published her seventh and final book in the series. The books have been published in 64 languages, sold more than 400 million copies and spawned a film franchise that has pulled in $4.5 billion at the worldwide box office.
Rowling is a fan of the Harry Potter Lexicon Web site that Vander Ark runs. But she draws the line when it comes to publishing the book and charging $24.95. She also says it fails to include any of the commentary and discussion that enrich the Web site and calls it "nothing more than a rearrangement" of her own material.
One of her lawyers, Dan Shallman, on Friday 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 made her own comparisons among her seven best-selling novels and the lexicon and was ready to testify about the similarities in dozens of instances.
David Saul Hammer, a lawyer for RDR Books, which plans to sell the lexicon, said the publisher will not challenge the claim by Rowling that much of the material in the lexicon infringed her copyrights.
But the judge will decide whether the use of the material by the small Muskegon, Mich., publisher was legal because it was used for some greater purpose, such as a scholarly pursuit.
In court papers filed prior to the trial, 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."
In court papers, Vander Ark, 50, said he was a teacher and school librarian in Byron Center, Mich., 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 Web site as a hobby a year later. Since then, neither Rowling nor her publisher had ever complained about anything on it, he said.
In May 2004, he said, Rowling mentioned his Web site on her own, 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 Web site for the dangerously obsessive; my natural home."
The Web site attracts about 1.5 million page views per month and contributions from people all over the world, Vander Ark said.
He said he initially declined proposals to convert the Web site into an encyclopedia, in part because he believed until last August that in book form, it would represent a copyright violation.
After Rowling released the final chapter in the Harry Potter series that same month, Vander Ark was contacted by an RDR Books employee, who told him that publication of the lexicon would not violate copyright law, he said.
Still, to protect himself, Vander Ark said he insisted that RDR Books include 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, Vander Ark still sounds like a fan, saying the lexicon "enhances the pleasure of readers of the Potter novels, and deepens their appreciation of Ms. Rowling's achievement."
But the affection no longer seems a shared experience.
In court Friday, Hammer said Rowling's lawyers did not want Vander Ark in the courtroom while Rowling testifies.