Warner Bros. and Rowling are suing RDR Books for copyright infringement for publishing Steven Vander Ark's unauthorized reference book, the "Harry Potter Lexicon," The Wall Street Journal said on its Web site.
"Should my fans be flooded with a surfeit of substandard books -- so-called lexicons -- I'm not sure I'd have the will or heart to continue," the Journal quoted Rowling as saying in court.
The report said she also described Vander Ark's book as "sloppy," "lazy" and "incorrect."
Rowling later issued a statement explaining it gives her "no pleasure to take legal action" against the publishers.
"I feel very strongly about an important issue that affects everyone and not just me," she said. "If books that plagiarize other works are permitted, authors, fans and readers stand to lose. There are lots of books in many languages that comment on or criticize 'Harry Potter,' and that's fine. But the book in this case is different. It provides no analysis and virtually no commentary. It takes far too much and it offers precious little in return."