The Viking Age, H. F. Du Chaillu, 1889.
The Hansa
The Hansa Towns, H. Zimmerman, 1889.
History of Commerce, Clive Day, 1913 (bibliography).
Civilization During the Middle Ages, George Burton Adams, 1918.
Cambridge Modern History, Vols. I and II.
Dutch Sea Power
Motley's Rise of the Dutch Republic (still the best source in English for political and naval history of the period).
History of the People of the Netherlands, P. J. Blok, trans. Ruth Putnam, 1898-1912.
History of Commerce in Europe, W. H. Gibbins, 1917.