J. K. Rowling may be no stranger to battles against evil wizards and magical beasts but in her latest tussle she is preparing to appear in person in the American courts.

The notoriously shy author is ready to testify herself to protect her ownership of Harry Potter against RDR Books, an independent American publisher, next month.

A US court is to decide whether she can block the publication of an unauthorised encyclopaedia of the wizarding world that she created.

Rowling and Warner Bros, the Hollywood studio behind the epic film adaptations of the Hogwarts adventures, are taking legal action to stop RDR from profiting from a Harry Potter Lexicon, a 400-page guide to the Potter books and movies.

Accusing the unauthorised book of lifting 2,034 of its 2,437 entries straight from her work, Rowling condemned the Lexicon as “a Harry Potter ‘rip-off’ . . . [that] interferes with my rights as a creator and copyright holder”.

She noted that RDR’s attempt “to co-opt my work for financial gain” was out of keeping with the spirit of the Potter stories, which are “full of moral choices and ethical dilemmas”.

She claims that the RDR book not only infringes her copyright but undermines her own planned lexicon. As she will donate its royalties to charity, “the losers in such a situation would be the charities that I hope, eventually, to benefit”.

The unauthorised encyclopaedia was compiled by a free internet website, The Harry Potter Lexicon, which claims 25 million annual visitors.

David S. Hammer, co-counsel for RDR Books, said: “The Harry Potter Lexicon draws material and inspiration from the Harry Potter series but is an entirely new piece of work. It is a companion to Rowling’s work, not a substitute for it. No one is going to buy the Lexicon instead of a Harry Potter book, or instead of seeing a Harry Potter film.”

Rowling, who is suing in the New York Southern District Court, in a case scheduled to begin on April 14, said that she felt “betrayed” by “a person who calls himself a fan”.

As a penniless single mother living in Edinburgh in the early 1990s, she struggled to find a publisher for the first book. Although worldwide sales of her seven titles have since topped 375 million, the pain of those early days has never left her.

She recalled them in dismissing RDR’s idea that fans could buy two encyclopaedias. It was “insensitive” to think that everyone could afford both, she said:

“While I am extremely fortunate now, having had periods in my life when I worried about having enough money to feed and clothe my daughter, it is obvious to me that many people do not have money to buy every book that appeals to them.”

Right to copy

In Britain and the US the work’s creator holds the copyright; this prevents substantial passages being reproduced without permission. In Britain the “fair dealing” provision allows use of copyright work for criticism and review. In the US rule, the third party must make a contribution in reproducing material.

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• The 100 Best Books of the Decade
• The Times picks the 50 best paperbacks of 2009
• The conversation: James Ellroy