As the creator of the world’s most successful Harry Potter website, Steven Vander Ark was the American high priest who presided over worship at the shrine of JK Rowling. He spoke at conventions, where other fans took his picture and asked for his autograph; he even left his job so he could devote more time to Pottermania.

Then, just over a year ago, the lecture invitations dried up. Friends began snubbing him and he started receiving death threats online. Suddenly he was about as popular as Lord Voldemort, the boy wizard’s deadly enemy. And all because Vander Ark, 50, had wanted to bring out a Harry Potter encyclopedia.

Now he is about to risk the wrath of Potter fans all over again by publishing a new book that matches real British locations to fictional ones in the novels.

The former school librarian lost a lot of his Potter credentials when Rowling applied for a court order asking for his first book to be banned. When the case came to court in New York earlier this year, she wept and was damning about the encyclopedia. “He has simply taken [my work] and copied it,” she complained. “It is sloppy, lazy and it takes my work wholesale.” The judge agreed, and the book had to be withdrawn.

Predictably, many online Pottermaniacs sided with Rowling. “It was all very hurtful,” says Vander Ark , who is still such a fan himself that he doesn’t blame the multi-millionaire author at all for her actions. “People were digging into everything they could find out about me - family, jobs, where I went to school. They even found a picture of a close friend, posted it on the internet, and attacked her weight and her hair.”

He was shocked when he greeted a pal in the street - only for her to ignore him and flounce off. Some contributors to internet chat rooms, he says, were even discussing plans to find him - “They said they wanted to get rid of me.”

By this time, he had come to London for a year to research his latest book. Yet he didn’t think of calling in the police. “I guess I didn’t think people would really come after me because it was an online world,” he says. “If there had been online police, I would have gone there.

“But I didn’t go out much. I hunkered down and just tried to protect myself. I was shocked to go from someone who signed autographs to being the worst person in the world.”

Vander Ark is one of life’s natural librarians. As a boy, he catalogued and cross-referenced episodes of Star Trek. In his late teens, he drew up blueprints for the Star Wars Death Star, night life presumably being quite slow in 1970s Michigan. He worked at a grocery store before deciding on a career change in his mid-thirties, then went to university and ended up working as a teacher librarian.

Ironically, his first David and Goliath battle was with colleagues at his ultra-Christian school, who believed that Rowling’s tales of ghosts and wizards were unsuitable for God-fearing young children. Vander Ark lost: the school library remains Potter-free. However, he was already totally hooked - and now claims to have read each book 40 times.

“I love keeping track of all the little details, and finding connections between things,” he says. “But I knew how much work it would take to catalogue a world that richly imagined.” So he didn’t crack until page 34 of the second book, Harry Potter and the Chamber of Secrets, where Mrs Weasley’s recipe books (Charm Your Own Cheese, Enchantment in Baking) proved too enticing to resist. “I grabbed a notebook, and just started taking notes.” As you do. By 2000, the notebooks had turned into a website: the Harry Potter Lexicon, which attracts 10,000 hits a day.

He has just endured a nail-biting wait as his latest book - In Search of Harry Potter (Methuen, £14.99) - was scrutinised by Rowling’s lawyers. In the end, they asked only for small changes on the cover. Vander Ark says that he now longs to meet Rowling - purely to say how much he regrets upsetting her.

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