Harry Potter superfan Steven Vander Ark wept in court on the second day of his legal battle with author J.K. Rowling, over his plans to publish an encyclopedia about the fantasy series.

The British writer sued Michigan, Detroit-based RDR Books for copyright infringement in 2007, in a bid to prevent the publication of Vander Ark’s Harry Potter Lexicon, based on a fan website he created, inspired by her novels.

Rowling has so far managed to stall the publication of the 400-page text, which was initially intended for release last November.

On Monday, Rowling told the New York court how the fan’s book "constitutes wholesale theft" of 17 years of her work - adding that her books were like "children" to her.

But, appearing in a U.S. District Court in New York on Tuesday, Vander Ark insisted he was just a big fan of the Harry Potter franchise, and had read every Harry Potter book "30 or 40 times" and every article ever written about Rowling.

Calling Rowling "a genius", the 50-year-old librarian broke down and claimed his clash with the author had left him an outcast among Harry Potter fans.

He told the court, "It’s been… it’s been difficult because there’s been a lot of criticism, obviously, but… that was never the intention.

"This has been an important part of my life for the last nine years or so." Vander Ark also claims he was initially reluctant to go ahead with the lexicon, but was convinced by RDR that it was legal.

However, a statement from Rowling and Warner Brothers - who own the rights to the Potter works - reads, "A fan’s affectionate enthusiasm should not obscure acts of plagiarism.

"The publishers knew what they were doing."

The case continues.