Internet History Sourcebooks Project
Fordham University: Medieval Sourcebook: Chronicle of the Fourth Crusade
Fordham University provides this site with information about Geoffrey de Villehardouin's chronicle of the Fourth Crusade, including the conquest of Constantinople.
Internet History Sourcebooks Project
Fordham University: Medieval Sourcebook: The Sack of Constantinople
This site from the Fordham University provides Niceta Choniate's first person account of the sacking of Constantinople by the crusaders of the fourth crusade.