Editor's note: Juergen Boos is director of the Frankfurt Book Fair. He gives us his views on the development of the e-book market.

We should view the success of e-books in the US market by acknowledging that the country has a unique approach to royalty payments, marketing and technology and customer habits. Moreover, in the US, romantic novels in e-book form have extremely good sales and outstrip any other type of literature. That's because of the demographic, whereby women aged 20 to 40 are the main consumers of e-books. So, despite the high sales in the US, the country should not be taken as a benchmark to evaluate the maturity of the world's e-book markets. For example, Germany is the world's third-largest market after the US and the UK, but sales accounted for just 1 percent of total domestic book sales in 2011.

Cultural differences are one of the main factors affecting the development of e-books. One of the reasons Japan is falling behind is that the country has no specified format and can't decide whether to use the horizontal or vertical version. In Brazil, e-readers are not popular because people prefer to read on their cell or smartphones.