Non-Bujinkan books:

The Gift of Fear by Gavin de Becker
This is a book that you need to read and then give to your friends and family members.  It's about listening to that little tickle in the back of your head that seems to say, "Things aren't quite right," then acting on that by leaving or putting a stop to a situation before it can develop into violence.

Marc MacYoung's books and Website:
www.NoNonsenseSelfDefense.com
Though we deal with self defense in class, most of what we do as martial artists is more about destroying an opponent than about sensible self defense.  Though I find the tone grating after a while, MacYoung & company generally do a good job in dealing with the issues surrounding violence and why you want to prevent and avoid violence rather than invite a host of headaches after the fact.  Some of the topics cover issues you may not have  encountered, but need to consider.

The Little Black Book of Violence by Lawrence A Kane and Kris Wilder.
If you've been studying for a while, most of this is common sense, but in case you're new to the game, a lot of it is very good material.  It is aimed at a young demographic.

Verbal Judo by George J. Thompson and Jerry B Jenkins.
These skills are essential, especially if you work in a field in which you are frequently dealing with difficult people.

The Unthinkable: Who Survives When Disaster Strikes - and Why by Amanda Ripley.
Very interesting reading.   Chances are that you will experience a disaster at some point in your life.  It doesn't hurt to think about it ahead of time.

What Cops Know by Connie Fletcher.
This one has been around for a while, but a good introduction to what is out there.

Crime Classification Manual: A Standard System for Investigating and Classifying Violent Crimes by John Douglas, Ann W. Burgess, Allen G. Burgess, and Robert K. Ressler.
Not the sort of thing I would normally be recommending, but it is eye opening.


Bujinkan Books:

Stick Fighting: Techniques of Self-Defense by Masaaki Hatsumi.

The Grandmaster's Book of Ninja Training by Masaaki Hatsumi.

The Way of the Ninja: Secret Techniques by Masaaki Hatsumi.

Unarmed Fighting Techniques of the Samurai by Masaaki Hatsumi.

Advanced Stick Fighting by Masaaki Hatsumi.

Essence of Ninjutsu by Masaaki Hatsumi.

Ninjutsu: History and Tradition by
Masaaki Hatsumi.

Knife and Pistol Fighting by Masaaki Hatsumi.

Ninpo: Wisdom for Life by Masaaki Hatsumi.

Beginning to Intermediate Taijutsu by Joseph Adriance.
The author is one of the gems of the Bujinkan.  I strongly recommend his book and any opportunity to study with him.