Sales of books, in both print and digital formats, are struggling around the world, despite the efforts to promote reading on World Book Day. However there is one area that's bucking the trend: Audio books. World Book Day is promoted a worldwide celebration of books intended to encourage reading, but revenue from book publishing is falling worldwide. Statistics from Euromonitor released in 2015 show that turnover from book publishing has experienced sharp declines in recent years. Between 2011 and 2014, revenue fell from $165 billion to $145 billion. The harshest decline was 2011-12, when the global book market contracted 7.4 percent.

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Some regions have recovered slightly since then. Data from the Association of American Publishers (APP) released this week showed U.S. book sales from January to October last year grew 0.5 percent, although overall revenue for publishers during the period was down 2.8 percent at $13.2 billion. Similarly, the U.K. market is doing better after a rough period, according to James Daunt, managing director of retailer Waterstones. "Book sales in the U.K. are in robust growth and have been for the last year," he told CNBC via email. "Physical book sales are growing at 3 percent to 4 percent after many years of decline. To give this context, the market for physical books declined by 25 percent in the three years of 2011 to 2013." Part of the reason for declines in print book sales was the rise of e-books and e-readers, but this sector is now falling. According to business magazine "The Bookseller", e-book sales by the U.K.'s five biggest publishers dropped 2.4 percent between 2014 and 2015, to 47.8 million.