The Catholic Encyclopedia 1913 ed. Be sure to look under other subjects too, like Luther, Calvin, Trent, etc.
The Oxford Encyclopedia of the Reformation. (Ref. BR 302.8 .O93 1996) This four- volume set is THE place to begin your research. The entries are written by scholars and include bibliographies.
Europe 1450-1789. (Ref. D 209 .E97 2004) This is a 4 volume set; a good place to begin your research.
Encyclopedia of Protestantism. (Ref. BX 4811.3 .E53 2004) This is a four-volume set; very scholarly, a good place to explore a topic.
The Reformation - Nice overview with contrasting differences at the bottom.
History of the Christian Church by Phillip Schaff.
The History of Protestantism by James Wylie.
N.B.: The above two books are over 100 years old.
Society for Reformation Studies - Good for listing of organizations and societies. Also see the Society’s home page.
Reformation History This site is good for the English Reformation
History of Protestantism Wiki essay
H-German - Good for discussions and book reviews.
The Reformation Timeline - Covers the English, Lutheran Reformations and the Thirty Years War.
DOCUMENTS AND PRIMARY SOURCES
Project Wittenberg - Good for writings of Luther and others. Some biographical too.
Martin Luther’s German Writings - Has a search engine--good for searching by word.
The Magisterial Reformation - This is good for select writings of Luther and Calvin and also useful for information on events and people.
Reformation - Good for selected writings of Luther, Calvin, Melancthon and Zwingli.
Internet Medieval Sourcebook - Good for critics of Catholicism before Luther and the Catholic Reformation.
The Catholic Reformation - Texts of Loyola and other writers.
Internet Modern History Sourcebook - In addition to the Protestant and Catholic Reformations, this site covers women and conflicts.
The Protestant Reformation - Good for Confessions other than Lutheran and the English and Scottish Reformations.