The inexorable rise of smartphones will pushed the valuation of the ebook business to $4 billion in the US alone in 2015, according to a new study by the market research company eMarketer. This finding is part of a wider study looking at how consumers are increasingly using their smartphones as a primary platform on which they buy and then consume content, and stacks up mobile content sales across various media types, including books, games and recorded music. What is perhaps most surprising of eMarketer's findings is that how well it expects books' total share of the mobile content to hold up against these other media types. With $4 billion of a US market valued at $10 billion, books are the single most important content category, ahead of other more 'digitally native' media, including both mobile games and online video. The report is also clear that mobile represents the future of content sales, predicting that by the end of 2015 nearly a third (31%) of the money American consumers spend on books, games, music or video will be spent using a smartphone. This eMarketer report also arrives at the same time as another in-depth look at the book business's relationship with mobile published by Digital Book World. In a 'virtual round table' discussion published on the DBW website, digital publishing consultants Thad McIlroy and Marcello Vena look at the opportunities presented to publishers presented by the two major competing mobile ecosystems, Apple's iOS and Google's Android. As we seen in previous blog posts and in the primary market research we commissioned last year, McIlroy and Vena both agree that Apple's iOS presents the lion's share of opportunity for ebook publishers at the moment. For example, while Android enjoys a significant lead in market share in some territories of Europe and the developing world, Apple commands the majority of consumer attention in the $10 billion mobile content market of the US. One graph that Vena deploys in his argument illustrates the scale of Apple's lead in this key territory. Despite Android's almost 10% market share lead over Apple in terms of the number of active devices in the US, Apple device users account for almost a quarter (24%) more mobile traffic. Quite simply, there may well be millions more Android phones and tablets active than their iOS equivalent, but Apple users use their devices more and spend more on them. McIlroy does offer one compelling counter-argument against publishers doubling down on iOS over Android. For those publishers seeking to grow digital sales outside of the US, where iOS market share trails Android by a large margin, focusing on content sales via Apple devices is not likely to help them to reach the largest addressable market. Where both Vena and McIlroy both agree, however, is that regardless of which mobile platform they might choose to favor, publishers face a huge uphill struggle when it comes to achieving their strategic aim of growing direct-to-consumer sales. As ebook sales in the US alone approach $4 billion, publishers entering the direct ebook sales market will find themselves competing in a mature market where millions of consumers are heavily invested in buying ebooks via their favorite reading platforms. Faced with the challenge of competing directly with Amazon's Kindle, Apple's iBooks or even Barnes & Noble's troubled Nook, publishers will, according to Vena and McIlroy, have to find a compelling reason to convince consumers to change their behavior.