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She spoke at a New York court yesterday in the case against Steven Vander Ark and publisher RDR Books, who she believes have violated her intellectual property rights.
The author said: "He has simply taken it and copied it. It is sloppy, lazy and it takes my work wholesale."
Ark's book, The Harry Potter Lexicon, is based on material gathered on a website of the same name.
Rowling also revealed that the case had left her "decimated" and claimed she was unsure whether she had the "will or heart" to continue with her own definitive Harry Potter guide.
Her legal representative Dale Cendali said that the publishers had copied Rowling's "life work" and simply placed somebody else's name on the title.
RDR Books argued that Rowling is trying to create "a monopoly" on the right to publish literary guides for her own novels.
Speaking before the case, Rowling said: "RDR's book constitutes a Harry Potter rip-off of the type I have spent years trying to prevent.
"If RDR's position is accepted, it will undoubtedly have a significant, negative impact on the freedoms enjoyed by genuine fans on the internet."
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