(FORTUNE Magazine) – IT'S HARD TO IMAGINE ANYTHING geekier--more supremely uncool--than a number puzzle. And yet people the world over are furrowing their brows over Sudoku. The game that has had Britain in its grip for the past year now appears in most major American newspapers and has spawned bestselling books, a TV show, computer programs, tournaments, and countless addictions. Not since the Rubik's Cube pandemic of 1980 or the crossword craze of 1924--25 has a puzzle generated this much madness--and unbridled commerce.

The central appeal of Sudoku (pronounced soo-DOH-koo) is its simplicity. The puzzle is a nine-by-nine square grid composed of nine three-by-three mini-grids. The goal is to fill the entire grid so that every row, column, and mini-grid has the numbers 1 through 9, each appearing once. "The craze, judging by history, will last four, five, six months, and then it will taper off," says Will Shortz, the editor of the New York Times crossword and high priest of puzzling. "But I think the underlying appeal of Sudoku will make it last forever. It's not just hype. If you do the puzzle, it's very easy to get hooked."

Shortz has three Sudoku books in stores now and will release eight more volumes by Christmas, all with St. Martin's Press. But printing presses everywhere are going into overdrive. Amazon.com now lists 82 Sudoku books, and not one was published before June of this year. One week in late September, according to Nielsen BookScan, the five top-selling Sudoku books together sold more copies than publishing juggernaut Harry Potter and the Half-Blood Prince.

It's not just the book industry that's seeing the ripple effects of Sudoku. The largest stationer in Britain, W.H. Smith, recently noticed that over the summer, sales of its two leading brands of pencils had gone up 700% and 300% at its airport and train station branches. The only conceivable reason, says the company: Sudoku.

The game was invented in America in the late '70s and migrated to Japan in the '80s. But the man behind Sudoku's proliferation is a retired judge from New Zealand named Wayne Gould, who discovered it while on vacation in Tokyo in 1997. Gould then created a computer program that could produce Sudoku puzzles. In October 2004, while Gould was in England, he appeared unannounced in the lobby of the Times of London and pitched the puzzle to an editor. One month later Sudoku hit the newsstands. More than 400 newspapers worldwide now carry it. Gould provides content to almost one-third of them at no charge as long as each puzzle appears with the name of his website, where he sells copies of his Sudoku program. He says he will make more than $1 million this year from Sudoku.

Alas, Gould worries that newcomers trying to cash in are publishing subpar puzzles. "I feel I'm in the role of the stepfather of this puzzle," he says. "I'm not the father, but I have popularized it, and I feel I owe it a duty to look after it." -- Jia Lynn Yang