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Oxford history of the Crusade, a very comprehensive book on the subject and a good introduction;)
 
Jean Richard's The crusades , in french or english, is the new textbook for the crusades and the latin kingdoms. Havard is right that runciman and norwich, while extremely good reads, are academically lacking.

those two 'warfare in history' books by Haldon and France are superb as is Riley-Smith's The Crusades and Mayer's History of the Crusades.

For crusading warfare the Smail book is still the best, although his opinions on the broader issues of the movement are outdated. Ian Heath's various books for WRG, like Armies and Enemies of the Crusades are good for the battles and the armies of the era on both sides.

In saying all that i still recommend the norwich books on byzantium, i read them as a child and thought they outdid any work of fiction for their engrossing style and imagery.
 
Two quite short -- but still scholarly -- books that give you a proper feel of the era:

Denys Hays "Medieval Centuries"
Henri Pirenne "Economic and Social History of Medieval Europe"

Gives you a good sense on how these kingdoms were run and what were the real conflicts of the day

(BTW: Both of them seem to agree that the Crusades played a very minor role. :) ).