Twenty years ago, an editor at Bloomsbury took a big chance on a very unusual book.

A debut fantasy novel set in 19th-century England, it told the story of two feuding magicians trying to revive the lost art of English magic. The unfinished manuscript was littered with elaborate footnotes that, in some cases, read like an academic treatise on the history and theory of magic. Its author, Susanna Clarke, was a cookbook editor who wrote fiction in her spare time.

The book, “Jonathan Strange and Mr. Norrell,” instantly launched Clarke as one of the greatest fantasy writers of her generation. Critics placed her in the pantheon alongside C.S. Lewis and J.R.R. Tolkien; some compared her sly wit and keen social observations to those of Charles Dickens and Jane Austen. Readers devoured the novel, which went on to sell more than four million copies.

“I had never read anything like it in my life,” said Alexandra Pringle, the former editor in chief of Bloomsbury, which commissioned a first print run of 250,000 copies. “The way that she created that world, a world apart from our world but absolutely rooted in it, was so utterly convincing and drawn with such precision and delicacy.”

The novel reshaped the fantasy landscape and blurred the boundaries with literary fiction, making the Booker Prize long list and winning a Hugo Award, a major science fiction and fantasy prize. Clarke went on tour across the United States and Europe, and Bloomsbury later gave her a hefty contract for a second novel.

Then, almost as suddenly as she appeared, Clarke disappeared.

Not long after the novel’s release, Clarke and her husband were having dinner with friends near their home in Derbyshire, England. In the middle of the meal, she felt nauseated and wobbly, got up from the table, and collapsed.