Mystery shopping is a qualitative market research method to measure the quality of customer service (including quality of products and services, as well as the professionalism and attitude of the staff). It is performed by mystery shoppers, who behave like average customers, then prepare reports including both positive and negative experiences. The method has been developed by the Mystery Shopper Provider Association (MSPA). The organisation has published detailed guidelines on the practical application of the method. A code of ethics has also been prepared, demanding professionalism, objectivity and honesty from the MSPA members. While shops are evaluated by mystery shoppers, libraries can expect mystery readers. The practical application of the method is widespread in the international library community. For example, libraries in New Zealand, UK and Germany are regularly visited by mystery readers. However, the method is not yet well-known in Hungarian libraries. The present article introduces a case study on 'mystery readers' which was carried out at the Semmelweis University Central Library. The aim of the study is to promote a market research tool which enables library users not only to be satisfied, but to remain loyal too.