Short answer: as much as you’re comfortable with.
Longer answer: … it’s complicated.
Not terribly familiar with the setting of BitD, so my advice comes from a generic position: the GM is the final authority over everything in her game.
This means several things:
- if using a published setting, the GM needs to decide how much “official” material she will accept as canon.
- if the players are familiar with the published setting, the GM needs to decide how much she wants to contradict their preconceived ideas. If she chooses to change or omit something, she should let the players know that things are different. If the players make a false assumption ~ because they read it in a book but the GM changed the details ~ she should give them the benefit of the doubt and inform them of the change, in the moment, and allow them the chance to rethink their choices.
- if using a home brew setting ~ or if she significantly alters the published to the point where it’s practically a homemade setting ~ she should make certain the players understand the key differences. Anything that the characters would know that the players do not, she should assume the character knowledge takes precedence and reinforce that knowledge (if and when it’s apparent that the players are making decisions based on bad assumptions).
The challenge with referencing the books during the game is that it can kill momentum. If you have a chance to look up something ~ to validate your memory or verify a particular assumption ~ try to do it when the players are talking among themselves. When you’re actively engaging them, running a fight or a tense situation, is the less ideal time.
If you anticipate using a particular detail in a given session and you’re concerned about sticking to published canon ~ as opposed to making it up ~ try and give yourself enough time before the session to refresh your memory, take notes, and recreate a web of relevant details.