I agree entirely regarding Sir Steven Runciman. Volumes 1 & 2 of his history of the Crusades are outstanding, as is his BYZANTINE CIVILIZATION. His book THE FALL OF CONSTANTINOPLE, 1453 is depressing, but what else could it be?
For many years he was head of The Friends of Mt. Athos, of which HRH the Prince of Wales was also a member. Their annual newsletter is quite interesting.