In an effort to encourage learning, research, critical analysis, I provide below a portion of my source material. Many of these are available in bookstores, or online.
Arnold, Benjamin, Medieval Germany 500-1300 : a political interpretation.
Brown, Peter R.L., The Rise of Western Christendom : triumph and diversity A.D. 200-1000.
Fouracre, Paul, and Davies, Wendy, Property and Power in the Early Middle Ages.
Ganshof, Francois Louis, The Carolingians and the Frankish monarchy.
Gibson, Margaret T. and Nelson, Janet L. (eds.), Charles the Bald : court and kingdom.
Herlihy, David, The history of feudalism.
McKitterick, Rosamund, The Frankish church and the Carolingian reforms, 789-895.
_________, The New Cambridge medieval history, vol 2.
_________, The Early Middle Ages : Europe 400-1000.
_________, The Frankish Kingdoms under the Carolingians, 751-987.
Nelson, Janet L., Charles the Bald
Reynolds, Susan, Fiefs and Vassals : the medieval evidence reinterpreted.
Wallace-Hadrill, J.M., The Frankish Church.
Etchingham, Colman, Church Organisation in Ireland, A.D. 650 to 1000.
Hughes, Kathleen, Early Christian Ireland: introduction to the sources.
Ó Cróinín, Dáibhí, New history of Ireland, vol 1: Prehistoric and early Ireland.
_________, Early medieval Ireland, 400-1200.
Madden, Thomas F., The Crusades : the essential readings.
Phillips, Jonathan, The Fourth Crusade and the Sack of Constantinople.
Cowdrey, H. E. J., The Crusades and Latin Monasticism, 11th-12th centuries.
Erdmann, Carl, The Origin of the Idea of Crusade.
Gregory of Tours, Decem Libri Historiarum/Historia Francorum (Ten Books of Histories/History of the Franks)
Bede, Historia ecclesiastica gentis Anglorum (The Ecclesiastical History of the English People)