The Pillars of the Earth, Ken Follet. Dodgy writing but brilliant evocation of the time, and especially social and engineering detail.
If you want to read about The Crusades seen from both sides, I can recommend "The Crusades" by Thomas Ashbrigde. It is about how it all started to the end of the Crusader States.
Also some about the Popes and their motivation for launching a crusade.
Barbara Tuchman's A Distant Mirror. A great history of the 100 Years War told from the perspective of an important baron (De Coucy, which if you notice is indeed a barony in the Paris county in the game).
It is in the Vermandois-county actually
A good book about the Baltic Crusades is: Eric Christiansen with 'The Northern Crusades'.
For historical novels, a couple of Bernard Cornwell's books were of this time period, and he's a very fun writer to read.