If you think that Christian lands and Muslim lands have only recently had enmity, this is the book that shows it’s not the case. Lars Brownworth’s In Distant Lands is an excellent exploration of what happened following Pope Urban II’s fateful speech in Clemont, France back in 1095. Europe would be mobilized to invade the East.
Their mission? To retake Jerusalem from the Muslim hordes. It appeared to be an impossible task given their lack of numbers and the cunning and brilliance of their enemy. Yet, despite this they would succeed in incredible circumstances. I think this may be the best couple of bucks that I’ve ever spent, and I don’t normally read history books.