It began as an audacious side project. Three dads and an uncle got together to make a personalized book for children. The Little Boy/Girl Who Lost His/Her Name, in which any child’s name, thanks to some nifty algorithms, dictates the plot turns, became a surprise hit. It was the bestselling picture book in the U.K. last year. This week, it topped a million copies sold worldwide (to actual customers, mind you, not retailers).

How do you follow up that sort of debut? Lost My Name, the London startup that grew out of the project—part tech company, part book publisher, and backed by Google Ventures and others—just launched its second personalized tale, The Incredible Intergalactic Journey Home. READ MORE: Is This The Most Technologically Advanced Book Ever Published? | FastCompany