Oxford Reference includes resources on archaeology, classical studies, history, philosophy, and religion, among others.

https://www.oercommons.org/curated-collections/800?batch_size=20&sort_by=title&view_mode=summary&f.sublevel=college-upper-division&f.sublevel=graduate-professional

Recommended websites for primary source material:

• Internet History Sourcebooks Project from Fordham University: www.fordham.edu/halsall
• Christian Classics Ethereal Library: www.ccel.org
• New Advent: http://www.newadvent.org/fathers
• Early Christian Writings http://www.earlychristianwritings.com/ 
• Early Church Fathers Additional Textshttp://www.tertullian.org/fathers/ 
• Fourth Century Christianity http://www.fourthcentury.com/ 
• History of Missiology http://www.bu.edu/missiology/ 
• The Labyrinth Church History  https://blogs.commons.georgetown.edu/labyrinth/categories/church-history/ 
• Post-Reformation Digital Library http://www.prdl.org/ 
• Project Wittenberg  http://www.iclnet.org/pub/resources/text/wittenberg/wittenberg-home.html  
• Church History Timeline http://www.historytimeline.org/