Perhaps a better title would be something like Don't Allow the Crusades to be Thoughtlessly Added to a Parade of Christian Horribles without Knowing More about It, but I wanted to get your attention.
Rodney Stark's God's Batallions is an outstanding book designed to help the educated reader (not only the academic reader) understand the Crusades. You know the routine. You want to talk about Christianity and the village atheist wonders just how you are getting past the horrors of the Crusades and the Inquisition. This book answers the question with regard to the Crusades. Stark brilliantly explains how the Crusades started, what happened in the course of events, and why they finally ended. All in all, the western church comes off pretty sympathetically. Readers who know Stark find it easy to trust him because he always questions excessive claims and makes sure to back his own assertions up with data.
What becomes clear is that the Crusades failed for three reasons.
First, despite the fact that the westerners regularly decimated their Muslim rivals in combat thanks to superior tactics and technology, they were always on the wrong end of a numbers game. The western armies arrived in the Holy Land already diminished from disease and harrying attacks along the way. They never had large enough armies to begin with. And whenever they secured their objectives, a substantial number of troops and/or nobles would return home leaving ridiculously small numbers to hold on, which amazingly, they did for decades at a time.
Second, Crusading was expensive. Although it has been suggested the Crusades were about wealth, nobles didn't get rich on them. They borrowed, scraped, and imposed heavy taxes just to be able to afford equipping, paying, and feeding their armies. When they captured an area, the land was not revenue-producing in the same way their European farm land was.
Third, the Byzantines never came through with the help they promised. Crusaders regularly expected help from the Comnenus family of rulers which began the Crusades by appealing to the pope for help. But the help was virtually never forthcoming. Had the Byzantine empire allied itself with the Crusaders, the Holy Land might still be in Christian hands today.
Read for yourself. I found the book highly enjoyable. Rodney Stark has reached the point to which many academics aspire. He writes about things that interest him for a mass audience with the aid of a major publishing company (Harper). And the books come to us rather than sitting staidly in university libraries.