The Bujinkan is where you study martial art. It is a place to learn how to fight how to defend and protect yourself your family and your community. You share this place - the dojo - with common purpose. Not only with fellow students, senior students, intructors, and master instructors but with the experience, stories, art, and practices passed on by warriors and Soke's from as far back as 34 generations. A Bujinkan dojo is a unique space.
The Bujinkan is not a religion and the dojo is not a place to meditate and come to an enlightened understanding on the meaning of life and the universe's creation. Nonetheless a curious student may wish to understand the contradictions that exist in an art that studies the practice of killing in the pursuit of peace.
However students are encouraged to read and do more in order broaden their understanding on life and the role they choose in such. You'll find many Bujinakan practitioners reading books on philosophy, spirituality, human behaviour, whilst also practicing yoga, other sports, going to the gym or other martial disciplines. There are many rock solid, open hearted individuals in the Bujinkan with years of life experience who have ears to listen - and if requested - offer suggestions on sources that may be of interest to you.
There is no prescribed reading however there are many books and dvd's written and produced by Hatsumi Sensei. It's a reasonable suggestion for a prospective or new student to add this material to their personal library.