For now, tablets like the iPad and Kindle Fire remain the most popular platform to read digital books. According to Nielsen, the percentage of e-book buyers who read primarily on tablets was 41% in the first quarter of 2015, compared with 30% in 2012. But what has captured publishers’ attention is the increase in the number of people reading their phones. In a Nielsen survey of 2,000 people this past December, about 54% of e-book buyers said they used smartphones to read their books at least some of the time. That’s up from 24% in 2012, according to a separate study commissioned by Nielsen.