There are a number of good introductory textbooks on the matter. My personal preference is Thomas Asbridge's, The Crusades (subtitled either "The War for the Holy Land" or "The Authoritative History of the War for the Holy Land" depend on which side of the Atlantic you are on). However, comparable general histories include Christopher Tyermann's God's War: A New History of the Crusades and Jonathan Riley-Smith's The Crusades: A short history or The Oxford History of the Crusades/The Oxford Illustrated History of the Crusades.

On this point though, Tyermann seems to have very recently published an extremely interesting sounding book that looks to be pitched at a general audience called How to Plan a Crusade: Reason and Religious War in the Middle Ages, though as I have seen no more than the cover and the publishers blurb I can't say for sure what it will cover or how accessible it may be.