Well, enough to read through it and give it a go.
My wife is a huge history buff (summa cum laude), and so I got this game for her as a surprise. For the FIRST TIME she is reading the book herself!
Get ready, Luke, cause my wife is picky! So excited!
What’s her area of specialty?
Medieval, particularly penitentials (she just reminded me of her thesis on Charlemagne in particular), but she has an unnerving talent for finding primary sources. Giving her an excuse to research anything is quite the risk at times 
GMing Burning Wheel for her can be a real nightmare, and is the reason I started reading your non-fiction sources in the first place. ( Her favorite response to any question I have about the Middle ages is “What year? What county?”)