Newly released is Armies of Heaven: The First Crusade and the Quest for Apocalypse by Jay Rubenstein. I haven't gotten a copy myself yet, so I can't review it yet. But in addition to this subject being of interest to a lot of folks out here, Jay is my wife's cousin, and I promised his mama I'd plug him to you guys! :)
Amazon link (http://www.amazon.com/Armies-Heaven-First-Crusade-Apocalypse/dp/0465019293/ref=sr_1_cc_1?s=books&ie=UTF8&qid=1319467997&sr=1-1-catcorr)