A screenshot of one of the books on offer through Worldreader Mobile

It sounds counterintuitive, but in certain developing nations it's easier to get hold of a cell phone than a good book.

More than one in three adults cannot read in sub-Saharan Africa, yet almost every home there has access to at least one mobile phone, according to USAID. Developing nations are among the fastest growing mobile markets in the world, but literacy is still a big problem.

David Risher, a former executive at Amazon and c0-founder of non-profit organization Worldreader, thinks there's an obvious answer here: offer free books through all those cell phones.

On Wednesday Worldreader Mobile comes out of "beta" mode. About a year ago it partnered  with biNu, a mobile app platform for feature phones in developing countries, so that its Worldreader Mobile app would appear on the home screen.

Richer says that in the last year, 10% of biNu's 5 million-user base have accessed the Worldreader Mobile app. That's about 500,000 people. Risher hopes to get that number to 1 million by the end of 2014.

Worldreader's biggest readers are in India (nearly 107,000 users), with 60,800 in Nigeria and 33,100 in Ethiopia.

Most of them are using low-end, pre-pay Nokia phones with physical buttons, that cost about $50. They will typically spend about $2-3 a month on their data plans. Out of the 6.4 billion active mobile phones in the world today, 5.4 billion are so-called features phones like these, according to Worldreader statistics sourced from Analysys Mason.

The books they're reading are short, typically taking up 150 screenshots. Though men are early adopters, women are the "power readers," Worldreader says, reading an average 17 books a month.

The most popular books are romance novels. Among the top five most popular books in the last month, the No. 1 was a children's book about school, the second an basic algebra book, and No.'s 3 and 4 were entitled My Guy and Can Love Happen Twice?

Risher says it's not unnatural in sub-Saharan African culture to see people hunched over reading their cell phones for long periods at a time. So pervasive have mobile device become that some elementary schools in Ghana have even started banning them, he notes.

Most of the books on Worldreader Mobile are in English, with a few in local languages like Swahili, French and Spanish, though the number of books overall is increasing. Currently there are 1,200 titles available, donated by local and English-language publishers.

"I think the single biggest thing that will get more people reading is putting more book on there," says Risher. "Everything form the Bible and Koran, to romance novels." Books on mobile phones are another way to get health information to people in rural communities, who often have access to pharmacies and relatively cheap, generic drugs, but encounter pharmacists with little sound medical knowledge.

How the Worldreader app appears on the biNu mobile platform

While accessing a book is free, it might cost users 5-10 cents from their data tariff to read. With the program, users don't download entire books but download a single page at a time, largely to save the cost of data and because low-end phones don't have much memory to begin with.

"The books are highly compressed," says Risher, adding that the biNu platform is designed to be thrifty with data consumption.

Readers need a constant 2G data connection to get through a book, but Risher says that basic cell phone coverage is pretty good across Africa. He sees a strong use case for spreading books and literacy through mobile phones, not least because his own service has been picked up so quickly.

"It's this classic case where the free market isn't going to solve this problem on its own. Books don't cost much, they're heavy, they go out of date," he says. "I've gone to Ghana and on the shelf of a school there I'll see the a book on the history of Utah. It's not only easier to get a cell phone, the books that do arrive are often completely irrelevant."

David Risher, CEO and co-founder of Worldreader; (image via Worldreader)

Risher founded Worldreader with his colleague Colin McElwee, in 2010, after Fisher spent time  volunteering in an Ecuadorian orphanage. Initially Worldreader donated Amazon Kindles to schools in sub-Saharan Africa, giving 3,000 children access to digital books. Part of the challenge of that program now is training teachers and children to use the e-readers.