Apostolic Fathers in various editions
A Library of Fathers of the Holy Catholic Church Anterior to the Division of the East and the West. Pusey’s project.
Ante-Nicene, Nicene, and Post-Nicene Fathers
Edited by Roberts, Donaldson, Schaff, and Wace
Ante-Nicene Christian Library
Patrologiae Cursus Completus
Edited by J.-P. Migne
Edited by Marguerin de La Bigne
St. John Chrysostom
The Works of Aurelius Augustine
Edited by the Rev. Marcus Dods, D.D.
St. Augustine: Miscellaneous volumes
Hard-to-find translations of the Church Fathers
Miscellaneous useful volumes
Italy and Her Invaders by Thomas Hodgkin. A huge and fascinating history from the beginning of the barbarian invasions to the coronation of Charlemagne, which necessarily touches on the ecclesiastical history of the times. Each chapter begins with a detailed list of sources, Hodgkin's critical analyses of which are alone worth the price (which, of course, is nothing).
Miscellaneous studies and sources
Mosheim's Institutes of Ecclesiastical History. "I am not ashamed to confess myself the disciple of the impartial Mosheim," says Gibbon, which is high praise.
The Catholic Encyclopedia.
The Jewish Encyclopedia.
Christian History in the Middle Ages.
The Reformation and Antecedents.