Anyone who has read any medieval European history knows about the Crusades, or thinks they do. But the whole period, from the First Crusade in 1096 to the Eighth in 1270, covers more than two centuries. The motivations of the Europeans in each case were somewhat different, the enemies they faced varied over time, and arms and strategy evolved through extended contact with the Middle East. The crusaders were able to establish a series of military states on the Levant that lasted for nearly a century, but which were ultimately doomed. And if the whole exercise was meant to be the "liberation" of the original seat of Christianity from Islam, well, that failed, too.
Nicolle is good at military history and he's a very good writer, but there's just so much you can do with a survey of so large a subject that is limited by the format of the book to a dozen pages. The following sections on the establishment and subsequent loss of the Crusader states, and on siege warfare, are also good but far too brief to be really useful. I began to think that perhaps Osprey should have broken this subject into three or four books in order to incorporate a useful amount of detail -- but a bit of subsequent research shows that they've done exactly that, with new volumes being published between 2005 and 2011. Richard Hooks color plates (one of the principal selling points in any Osprey book) are quite good, by the way, and cover the whole period.