Benjamin Whitmer brought my attention to this essay on self-defense by Sam Harris. It's a fairly good piece, and one of the books Harris recommends, Meditations on Violence, is excellent.
As a martial artist, and as a former boxer, and as someone who has been involved in too much street violence, I will say that it is impossible to train for a real fight - and the best preparation is understanding that.
Although most of Harris' advice is good, he gives one piece of advice that is so wrong-headed as to be dangerous:
What if a person dressed as a police officer comes to your door and asks to be let in? Unless you are absolutely certain that he is a cop—e.g. you can see that he arrived in a marked police car—you should explain that you have no way of knowing who he is and then call the police yourself. Thousands of crimes are committed each year by people impersonating cops.
Effective Self-Defense Includes Never Talking to Cops
The actuality is that thousands of crimes are committed by cops, not just by people impersonating them. Harris' perspective is that of a privileged white man who trusts uniformed authority. (That's why he thinks authorities should be able to torture people.) You should never let a cop inside your home unless they have a warrant and you can't stop them. If you know they really are cops, and you (foolishly) insist on talking with them, you should go outside and lock your door behind you.
But, again, you should never talk to cops. This video, which I've posted before but is well worth repeating, explains why.