"We all know I've made enough money," said J.K. Rowling, the author, in a courtroom in New York City this week. "That's absolutely not why I'm here."

It is not obvious, though, why she was there. Along with Warner Brothers Entertainment, Ms Rowling is suing RDR Books, a small, respected publisher in rural Muskegon, Michigan, to enjoin it from selling an A-to-Z guide to Ms Rowling's seven Harry Potter books. This "lexicon" would be based on an online guide compiled over the past decade by Steven Vander Ark in the hours he could spare from his job as a librarian at a Christian junior high school near Muskegon.

The case has become a battle over the limits of copyright law and a window on the ties that bind celebrities to their more obsessive fans.

The gravamen of Ms Rowling's and Warner Brothers' argument is clear. Mr Vander Ark's book "is not a reference book or scholarly critique", they claim, and it lacks "any originality or invention". Ms Rowling has praised Mr Vander Ark's website, but calls the book that will draw from it "wholesale theft". Her attorneys note that "Ms Rowling has allowed fans and scholars wide latitude to comment on, critique, and even create 'fan fiction' and art based on her stories". But of course, nobody in a free country requires authors' permission to comment on or critique their work.

Lawyers at Stanford University Law School's Fair Use Project, who are defending Mr Vander Ark pro bono, sought to show in three days of testimony this week that the Lexicon constitutes "fair use" of Ms Rowling's work. It is a reference guide, of the sort that is familiar (and indispensable) to anyone who has taken a deeper interest in Balzac, Proust, Faulkner or Star Trek . Ms Rowling "appears to claim a monopoly on the right to publish literary reference guides and other non-academic research relating to her own fiction", according to Mr Vander Ark's lawyer. Joe Nocera, The New York Times business writer, puts it even more bluntly. He has called Ms Rowling a "copyright hog".

Whether the lexicon violates "fair use" depends, according to US legal experts, on whether it is "transformative" or whether it just cribs from Ms Rowling's plot and prose. Much of the testimony missed this issue. Ms Rowling dwelt on her own plans to publish a Potter encyclopaedia, which is neither here nor there. Literary critics cannot be kept from writing about, let us say, the novels of Philip Roth on the grounds that Mr Roth swears he wants to publish a book called What My Novels Mean . The fact that Mr Vander Ark would profit from his lexicon is a red herring, too. Provided he is within the boundaries of "fair use", there is nothing illegitimate about his profiting from his work, any more than it is illegitimate that book reviewers be paid if they cite the books they review. Ms Rowling also demeaned the quality of Mr Vander Ark's book, which is legally irrelevant. Apparently some puns she was particularly proud of, including a "double allusion" in the name Remus Lupin, went over his head. She came off as condescending ("It's very difficult for someone who is not a writer to understand"), self-involved (the suit, she said, "has really decimated the demands of my creative work for the last month") and mean.

Meanwhile, Mr Vander Ark's admiration for Ms Rowling's "genius" (his word) remains slavish. Although he refers to her as "Jo" on his website, the two had never met before this week. He has read the Potter books dozens of times and recently moved to England, where he has written a book about the places that inspired the Potter series. Even after Ms Rowling filed the suit against his publisher last October, he signed his online postings "still Jo's man, through and through". When asked in court this week if he still felt like a member of the "Harry Potter community", he began to sob.

Whatever the court decides on legal grounds, one need only spend five minutes at Mr Vander Ark's website ( hp-lexicon.org ) to see that, on literary grounds, the idea that he is merely cribbing is nonsense. The website is highly transformative. It is a leviathan effort of research, criticism and interpretation. It is a concordance, index and bibliographical essay all in one. If the eventual book bears the slightest resemblance to it, it will be indispensable to scholars and lay Potter addicts. It gives timelines of the novels and points up inconsistencies in them. Its section on plants describes the uses and behaviour of fluxweed, honking daffodils and whomping willows, and reconstructs seven years of the "herbology" curriculum at Hogwarts. It indexes everything Ms Rowling has ever said in published interviews about her main characters. (At World Book Day in 2004, if you'd care to know, she hinted that Harry might be a relative of Godric Gryffindor, the wizard and Hogwarts founder.) It links to 137 literary essays about the Potter series from around the English-speaking world, some of them superb. It debunks hoaxes and rumours that have swirled around the series and its author. It links to bookstores. Such a site is not just a godsend to Potter addicts. It is thanks to readers such as Mr Vander Ark that Harry Potter is taken as something more than just a particularly good children's book.

Celebrities and their fans exist in an awkward mutual dependence. Maybe Ms Rowling, paradoxically, is a victim of her relatively private lifestyle and her unlikely road from single motherhood to fame. She has a Hollywood star's relationship to her public, but less practice than the average Hollywood star, perhaps, in hiding impatience. She remains a writer with an admirable work ethic, a magnificent gift for characterisation and plot, and a rich and inventive vocabulary - even if the word "humility" does not figure in it.

The writer is a senior editor at The Weekly Standard