Victory in the East by John France. It is the history of the First Crusade solely from a military view.
Very interesting from the military historian's viewpoint, and it indirectly sheds light on the Crusaders conduct in general. For example, one argument I'm sure you've heard many times is that the Crusaders couldn't have been pious men if they were starving out entire towns. Well, it turns out that they practiced the "military science" of times in the Middle East pretty much the same way they were practicing it on themselves in Western Europe at the time. Higher beliefs had nothing to do with the way they behaved, slaughtering peasants (who fed the cities) was simply the way warfare was conducted in 1095.
Interesting to know. Thanks for the suggestion.