The library of the Order of St Augustine in the Lesser Town of Prague, comprising more than 18,000 volumes, is an extraordinary book set of significant scientific and historical value. Nevertheless, a scientific study that would cover the entire history of the library was not available until now. This article provides a brief outline of the history of the monastic library at St Thomas, mentioning some remarkable figures that have contributed to its prosperity. The introductory part brings information on the binding rules governing the librarian practice in the monastic libraries of the Order of Saint Augustine in the Middle Ages. Because of the long time span, this treatise has been divided into two parts – the medieval and modern history of the library. Particular attention is paid to two periods – the 14th and 18th centuries, during which some members of the Order held leading positions at the university in Prague and, through their pastoral, preaching and educational activities, they were actively involved in the social, cultural and spiritual edification of broad social classes.