Rowling said she had planned to write her own Potter encyclopaedia
JK Rowling has told a New York court that plans to publish an unofficial Harry Potter encyclopaedia amounted to "wholesale theft".
She said she had stopped work on a new novel because her legal concerns had "decimated my creative work".
"I really don't want to cry," she said as she gave evidence in her copyright infringement case against writer Steve Vander Ark and his publisher RDR Books.
The case is expected to last most of the week.
Ms Rowling, who denied the case was about money, accused Mr Vander Ark, a librarian, of "an act of betrayal" in using her fiction as the basis for his guide.
JK Rowling speaks to reporters in New York
"He has simply taken it and copied it," Rowling told the Manhattan courtroom. "It is sloppy, lazy and it takes my work wholesale."
The author said she is not sure if she now has "the will or the heart" to write her own definitive encyclopaedia, the proceeds of which she had intended to donate to charity.
Earlier, Ms Rowling's lawyer said RDR Books, based in Michigan, was trying to scoop her plan to publish her own guide to the Harry Potter universe.
Dale Cendali told US District Court Judge Robert P Patterson that RDR Books had copied Ms Rowling's "life work" and put them in a book bearing "the name of somebody else".
The Harry Potter Lexicon is based on material from an internet fan site of the same name.
'A Harry Potter rip-off'
Ms Rowling, who claims the book violates her intellectual property rights, brought her legal case against Mr Vander Ark and his publisher last year.
In court papers filed before the trial, the author said she was "deeply troubled" by plans to turn the website into a book.
"RDR's book constitutes a Harry Potter rip-off of the type I have spent years trying to prevent," she said.
"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.
RDR Books has accused Ms Rowling of seeking to "claim a monopoly on the right to publish literary reference guides, and other non-academic research, relating to her own fiction".
If accepted, it said, her injunction "would dramatically extend the reach of copyright protection and eliminate an entire genre of literary supplements - third party reference guides to fiction".
The case continues.