Here's something for him to start with:
A History of the Crusades: Volume 1, The First Crusade and the Foundation of the Kingdom of Jerusalem (Cambridge University Press 1951)
A History of the Crusades: Volume 2, The Kingdom of Jerusalem and the Frankish East (Cambridge University Press 1952)
A History of the Crusades: Volume 3, The Kingdom of Acre and the Later Crusades (Cambridge University Press 1954)
I read the third one and have to go back and acquire (and read) the first two. Kind of dry, but a must-read for any Crusader historian or enthusiast.