Certainly encompassing a larger time period than just the 16th Century, and thus requiring some more targeted searching by limiting search criteria date ranges, this database gives researchers access to primary source material.
Early English Books Online (EEBO). [Proprietary]
Contains about 100,000 of over 125,000 titles listed in Pollard & Redgrave’s Short-Title Catalogue (1475-1640) and Wing’s Short-Title Catalogue (1641-1700) and their revised editions, as well as the Thomason Tracts (1640-1661) collection and the Early English Books Tract Supplement.
This resource searches the holdings of the Meeter Center for Calvin Studies, a research center that seeks to hold in print a copy of every possible piece of research, published or otherwise, on the Reformed branch of the Reformation, but they also collect materials more broadly related to the Reformation. Search results yield location and call numbers related to their particular collection. Resources are not available digitally. This database is best viewed, therefore, as a searchable bibliography for the identification of resources to then be obtained through other means.
Oxford Encyclopedia of the Reformation. [Proprietary]
The complete four-volume reference work mentioned elsewhere in this pathfinder is here available online. Article bibliographies are included. Cross-references are hyperlinked. Only keyword searching is possible.
The Post-Reformation Digital Library (PRDL), is an indispensable webliography of digital resources from digital libraries in Europe and North America. A very helpful metaguide, though it requires a healthy measure of prior knowledge to navigate the collection effectively. Use the sidebar index for quick access.