Smail's crusading warfare is the basic text for crusading military history. It's a classic.

Note that it only covers the period up to about 1193; a later work by another author (Christopher Marshall, Warfare in the Latin East, 1192-1291) covers the later period. It's pretty good as well.

John France's book on the First Crusade is great. (I met him once at a conference-- interesting guy.) I highly recommend it.