I would recommend any number of works by Jonathan Riley-Smith for that topic. He's written fairly extensively on the topic so you have a decent few books to pick from. I'm only really familiar with The Crusades: A Short History which won't be specific enough for you but I've read several of his articles as well. The First Crusade and the Idea of Crusading and The First Crusaders, 1095-1131 would both be great starting points.

The only caveat I'd put on Riley-Smith is that he generally takes a more positive view on the motivations for the Crusades than some historians. I'm not suggesting he's wrong in doing so, he's made some very convincing arguments to support his theories and is one of the most respected historians in this field, instead I'm just pointing out where his bias lies so you know ahead of time.