A professor in St Louis, MO is the most respected author on the Crusades that I know of. He has no AX to grind. His name is Thomas Madden.
I've read some of his writing...and listened to him interviewed and he really does a fine job of clarifying what some biased historians had done to twist the truth...good and bad...about the Crusades.
The Catholic church did offer heaven to those who went on the crusades. That's just plain history. And they wanted to free Jerusalem especially from the "militant muslim hordes". Sound familiar to the headlines today?
Anyway....Madden is not a believer or a Jew that I know of. Very scholarly.
Karl