Scholastic is launching an original e-book, The 39 Clues: Cahill Files: Silent Night, on Christmas Day. The title follows greater-than-expected sales of The 39 Clues: Rapid Fire, a series of original e-books released last Christmas season.

While last year’s effort helped answer readers’ questions about plot and characters within the franchise, this year’s is a standalone story featuring an ancestor of one of The 39 Clues’ main characters. It tells the true World War I story of the Christmas Truce of 1914, during which members of the opposing armies laid down their arms, played soccer, and celebrated the holiday together.

“It’s a sweet Christmas story and a holiday gift to the fans,” says Rachel Griffiths, executive editor. The title will be available for $3.99 through online retailers and will include a bonus collectible digital game card as well as the first chapter of Infinity Ring #1: A Mutiny in Time, the inaugural title in the latest multiplatform series from Scholastic. The second title in that series released this month.

The 39 Clues: Rapid Fire, a story released as seven daily e-books (priced at 99 cents each) during Christmas week last year, continues to sell, according to Stacy Lellos, Scholastic’s v-p marketing and multiplatform publishing. “It was conceived with the idea of being a gift to our fans, but the analytics on our website show that it’s bringing in new fans as well,” she says, noting that 1,200 kids register on the 39 Clues Web site every day.

Scholastic is launching a new cycle in the franchise, The 39 Clues: Unstoppable, in October 2013, after concluding the latest arc, The 39 Clues: Cahills vs. Vespers, this coming March. The entire franchise, which launched in September 2008, has more than 15 million copies in print worldwide and more than 2 million registered users of its companion online game.