The Catholic Encyclopedia 1913 ed. Be sure to look under other subjects too, like Luther, Calvin, Trent, etc.
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. Recheck link
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. Recheck link
The Protestant Reformation - Good for Confessions other than Lutheran and the English and Scottish Reformations.