First Crusade by Thomas Asbridge
I highly recommend this book and its companion The Fourth Crusade by Jonathan Phillips. It gives a good overview of how Gregory VII and Urban II changed the dynamics of the Church and managed to create a war of ideology (not unlike the current situation).
If you feel reading two books on the crusade is too much I highly recommend The New Yorker review of these two. Be sure to check on Doge Enrico Dandolo, one of the craftiest men in history.