This is not a source, but a search engine. It is a tool for locating open-access books, journals, dissertations and other scholarly sources. It is sponsored by the Bielefeld University Library in Germany.
The Post-Reformation Digital Library (PRDL) is a select database of digital books relating to the development of theology and philosophy during the Reformation and Post-Reformation/Early Modern Era (late 15th-18th c.). Late medieval and patristic works printed and referenced in the early modern era are also included. The PRDL is a project of the Junius Institute for Digital Reformation Research.