The Gotha Research Library preserves a top-class collection on the cultural history of Protestantism in the early modern period, which was compiled by the Dukes of Saxony-Gotha. An essential component of this unique handwritten tradition, which reaches far beyond the Central German cultural area, is the estate of the theologian and church historian Ernst Salomon Cyprian (1673-1745), one of the three main representatives of late Lutheran Orthodoxy. The estate comprises German and European sources of the history of the Reformation and historiographical sources of the early 18th century. It is untapped and largely unknown to research, so it is to be made accessible for the first time. In terms of content and methodology, the project follows the cataloguing of the Gotha Reformation manuscripts (16th century) and the post-Reformation estate of the theologians and polyhistors Johann and Johann Ernst Gerhard (17th century), which were funded by the DFG (Deutsche Forschungsgemeinschaft - German Research Foundation), and completes the first fundamental survey of the core holdings of the research library on the history and reception of the Reformation in the early modern period. The aim is to develop uniform descriptions with a high level of detail of the sources handed down in the research library and, in small parts, in the Landesarchiv Thüringen - Staatsarchiv Gotha (State Archives of Thuringia and Gotha). They are to be made accessible in the online databases of the library and the archives, in the portal Kalliope as well as in a printed catalogue and form the basis for further research and digitisation projects.