Traditional editorial houses are nowadays confronted to the emergence of online diffusion of e-books. This phenomenon forces publishing houses to make their catalogue of printed books available also in a digital version. Nonetheless, not all the publishers are on the way for publishing in digital formats. More specifically, in recent years many small digital editorial houses (i.e. selling only e-books) are appearing on the market, but most of the traditional book publishers seem to be reluctant to make available their complete catalogue in digital formats.