In what is believed to be a first for an e-book, “The Girl With the Dragon Tattoo,” by the Swedish novelist Stieg Larsson, has sold more than a million copies in its digital format, Knopf said on Thursday. Knopf, part of Random House, said the combined digital sales for all three books in the blockbuster “Millennium” series, which includes “The Girl Who Played With Fire” and “The Girl Who Kicked the Hornet’s Nest,” is now more than three million copies. The trilogy is selling at a clip of more than 500,000 copies a month in all formats. “The Girl With the Dragon Tattoo,” which went on sale in the United States in September 2008, has sold more than 300,000 copies in hardcover.

No other major publisher said it had yet sold one million e-books of a single title. Several publishers said they had individual authors, like Nora Roberts of Penguin Group USA, who has sold more than a million digital copies of all her combined books.

The “Millennium” trilogy has sold nearly 17 million copies in the United States.