As the soaring popularity of e-books reshapes the book business, publishers and authors seem to be sitting on a big potential resource: the millions of out-of-print books still unavailable on e-readers.

Just how big is the opportunity? A new study by researchers at Carnegie Mellon pegs the figure at more than $400 million. Using data from sales of out-of-print books already converted into e-books, the Carnegie Mellon researchers created an algorithm to predict future sales of the 2.7 million titles not yet digital. Their reasoning: Even if sales are slow, once books are digitized, offering them continually to the public will cost almost nothing, a big contrast from the traditional cost of printing books and selling them in brick-and-mortar stores.

But the book industry faces a conundrum in getting these books digitized. Unlike typical back issues of newspapers and magazines, the legal status surrounding old books can be murky. In some cases ownership has reverted to authors, in others new royalty rates need to be negotiated, and in still others it's simply unclear. "Every contract is unique," says publishing consultant Mike Shatzkin.

The study predicts theoretical sales of $740 million in the first year -- with about $460 million accruing to publishers and authors. (For context, that's nearly 3% of the publishing industry's total $27.2 billion revenue in 2011, according to the Association of American Publishers.)

"You have huge untapped potential," says Rahul Telang, who co-wrote the study with Michael D. Smith and Yi Zhang.

While no one is predicting huge sales for every old potboiler, there have been some surprising success stories. Last year romance author Barbara Freethy topped the New York Times e-books best-seller list after reissuing more than a dozen of her old titles electronically. She has since sold about 2.7 million copies of these and a handful of new works. "I found a whole new readership," she says.

While Freethy's case may be unusual -- she got the rights back from publishers and reissued them herself -- big name publishers are scrambling to follow suit, experts say.

HarperCollins, which has already published 23,000 of its titles as e-books, says it is working hard to make older titles available, despite the sometimes painstaking legwork. "It's a book-by-book, contract-by-contract process," says a spokeswoman.