To understand the motivations of those who see religious considerations as a justification for conflict it is useful to have a clear view of the Crusades, when such considerations were a significant part of European life. Contesting the Crusades
by Norman Housley offers such a view. He considers what might be classified as a crusade, looking at the importance of Jerusalem to medieval Europeans, and at how wars on other fronts were put into the form of crusades. There's a lot of information in this short book, and I would recommend it to anyone with an interest in the relationship between war and religion or in the history of medieval Europe.
This is a historiographical work - it looks at the work of previous historians, considering how personal bias may have influenced their work and what interpretations have been popular at different times. Housley considers the motivations for the crusades - were they just cynical manipulation of the people by the church, or were they genuine expressions of religious devotion by the general population? The book seems primarily aimed at an academic readership, but I feel that it is clear enough to be read by the interested non-academic reader, although I think some previous knowledge of the history of the Crusades would be very useful.