I just finished reading this reference last night. Wow! For one, the Crusades were a empty spot in my knowledge of the Middle Ages, and this book filled that in nicely. I knew that there were several Crusades, and that King Richard and Saladin played a part, but that was about it.
This work, without being burdensome, acts as a great primer to those events, and the players. When finished, you'll be ready to run an AD&D campaign in Outremer... the Crusader States in the Middle East. The guide details 3 levels of camapigns: Historical, Legendary, and Fantasy. You decide how realistic, or how cinematic to run your campaign.
The Crusades are a fine background for FRPG play, with medieval cultures mixing, trading, and fighting. Are you a Frank, fresh off the boat? Are you a second generation European living in Antioch, gone native? Are you a Saracen, trying to get these foreigners expelled from Muslim lands?
The guide doesn't flinch from the realities of brutal medieval warfare, and helps the DM deal with these issues. Both Frankish and Muslim culture and perspectives are addressed, as well as the situation for women. Several adventures and campaign seeds are introduced to get you started.