I just finished reading two books for a review - The First Crusade: A New History, by Thomas Asbridge; and The Fourth Crusade and the Sack of Constantinople, by Jonathan Phillips. I highly recommend both - and in fact, I recommend reading both back-to-back.
The First Crusade was, from the point of view of the Christian West, a miraculous victory - against seemingly insurmountable odds, the Crusaders achieved their goal of liberating Jerusalem from the Muslims. The Fourth Crusade was an appalling and shameful failure - crusaders bent on freeing Jerusalem (it had fallen into Muslim hands again in the interim) instead turned on the Eastern Orthodox city of Constantinople, raping and murdering their fellow Christians.
Reading both books together allows you to trace the development of the crusading message in the eleventh century to its inevitable and terrible conclusion in the thirteenth - and of course, historical conflicts between Islam and Christianity have an even more pressing relevance today.