I am currently reading a version of this book, which also contains The Book of Family Traditions on the Art of War by Yagyu Munenori. These are both books on the martial way.
The part I am on now (from Yagyu's book) is talking about "The Great Learning." Whenever you go into a house, you go through the gate to get to the house. Learning is the gate, not the house. Learning is the gate to the attainment of the way.
Miyamoto mentions something to the effect that training your body and learning movements are not enough... you have to train your whole self, your mind and heart, your understanding and awareness.
Even though the authors come from quite different backgrounds, these books seem to go well together. I don't find it an easy read, but the individual concepts themselves are not too tough to comprehend.