App-based novels remain a relatively new and unproven format, but they could begin to catch on as some prominent authors experiment with the interactive possibilities of apps.
Last year, the British novelist Iain Pears released his new book "Arcadia" as an app that allowed readers to toggle among 10 different story lines, and Eli Horowitz published his comic dystopian novel "The Pickle Index" as a hardcover, paperback and interactive app simultaneously. This spring, the best-selling novelist Wally Lamb will publish a new novel, "I'll Take You There," exclusively as an app via the digital publishing company Metabook.
With its episodic delivery schedule, "Belgravia" will test whether "appointment reading" can become as habit-forming as a recurring TV show or serialized podcast. Jamie Raab, the president and publisher of Grand Central Publishing, said the concept behind "Belgravia" appealed to her because of Mr. Fellowes's television background and his knack for keeping audiences engaged in a story over months and even years.
"I've always been intrigued by the idea of publishing a novel in short episodic bites," she said. "He gets how to keep the story paced so that you're caught up in the current episode, then you're left with a cliffhanger."
The thematic overlap with "Downton Abbey," and the timing of the release, as "Downton Abbey" comes to an end, could help attract fans of the show who are experiencing withdrawal, she said.
" 'Belgravia' deals with the different classes in England, and I think that's what people like so much about 'Downton Abbey' and before that, 'Upstairs Downstairs,' " she said, referencing a show that ran in the 1970s. "There's a lot of drama and melodrama."