Non-Bujinkan books:
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:
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. 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 haven't encountered, but need to consider.
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.
These skills are essential, especially if you work in a field in which you are frequently dealing with difficult people.
You never know...
This one has been around for a while, but a good introduction to what is out there.
Very interesting reading.
Bujinkan Books:
Masaaki Hatsumi.
Masaaki Hatsumi.
Shihan Joseph Adriance is one of the gems of the Bujinkan, but he's kept a fairly low profile, so you just don't hear about him all that often. I strongly recommend his book and any opportunity to study with him.