The book takes as its starting assumption that you know a vague outline of the Crusades - the sort of pop-culture thing that one picks up almost by osmosis (well, I did, anyway) - Christians v. Muslims, fighting over Jerusalem, several different Crusades with varying degrees of success, Richard the Lionheart (absentee king of England) v. Saladin (the Muslim leader who was nonetheless the epitome of a chivalric knight), bloodshed & brutality on both sides for the supposed glory of God (whichever one you believed in). And then by clearly discussing what actually happened, with a lot of reference to the sources of the time on both sides, the book shows how this concept of Crusade is both true and not true at all.
I'd not really realised before reading this book that even the only successful Crusade in the Holy Land (the First Crusade) only succeeded by chance - the Muslim world wasn't united and didn't really see the Crusaders as an actual threat until too late, and even afterwards it took a few decades for momentum for jihad against the occupiers of Jerusalem to build up. And the start of the Crusades was really that the Byzantine Empire wanted a hand getting its territory back & Pope Urban saw a chance to unite the warring aristocracy of Europe by getting them to fight somewhere else not against each other. And then the whole thing snowballed certainly beyond what the Byzantine Empire was after if not past what Pope Urban envisaged.
The author uses sources from both sides to give a flavour of the cultures involved in the various conflicts. And where possible concentrates on individual people, which makes it much more interesting and human than a birds eye view of the politics. There are lots of contradictions in even the Holy Land crusades - one crusade ended up sacking Constantinople, a major Christian city, for instance. And Richard the Lionheart got on well with Saladin's brother, possibly even was friends with him, certainly better than with some of his fellow European monarchs. Those sorts of things and also the overall sweep of the history make it clear how much depended on the individual people, how much was determined by the personalities involved on both sides, in the wars and in the peace. As it does with all history, I guess, but it can be easy to lose sight of that in books which concentrate on overall trends and overviews of a culture/historical period.
As well as the Holy Land crusades the book also covers the other crusades that expanded "Christendom" into Hungary & other parts of Eastern Europe and also the Re-Conquest of Spain. Both of those were more successful than the Holy Land crusades were overall, but less glamorous I guess so haven't made it into the pop-culture "Crusade history" (except in Spain, I imagine?). And he finishes up with a chapter looking at how the idea of Crusade has come down through history since - in both the East and the West, and how that affects the modern world.
I thoroughly enjoyed reading this book - maybe not everyone's idea of a good "beach read" but definitely was mine :)