This is the type of book that people outside of combat zones, especially pure civilians, should not need to own or read because the information it provides is only applicable to a specific circumstance: warfare where the enemy has no uniform.
"Bang" being an event like a sniper ambush, a knife strike, an IED detonation, or a suicide bomber attack, and staying "Left" means prior to the event if you were to draw the series of circumstances out on a time line. To do this you must get to something common among all humans - human nature.
In older conflicts between two organized militaries with different uniforms, different sounds from the distinct weapons they used, and even sometimes racial differences manifesting in the appearances of the soldiers, you could distinguish between the "good" and "bad" guy in most every case by judging on just those obvious physical distinctions.
With 4th Generation Warfare (4GW), there is an asymmetry in that there is usually one organized military and one disorganized. The soldiers in 4GW for the organized side are essentially on defense, because the other side is an insurgency, a rebellion, an anti-something that doesn't have numbers to partake in open conflict, but guerrilla type warfare, terrorist attacks, demoralization, these things require few people and little planning and organization. There isn't budget, or desire, to get everyone wearing the same outfit, using the same weapons, or even being of the same nationality.
The more structure and commonality they adopt, the more angles of attack they open up on themselves. Chaos is their ally, adaptability their MO, and the old rules just don't work anymore, and in fact those rules serve as a guide on where the easiest fault lines are to exploit.
For example, in the United States there are (for the sake of simplicity) rules saying that people cannot prevent other people from speaking their minds and practicing their religion. While at first these rules seem sensible, in practice this means that if one wishes to sow seeds of discord, then speech and religion are reliable vectors, because discrimination is supposed to be against the rules.
People can't silence you, and if you are prevented from practicing your religion, you point to how others are not being restricted and how the rules must either be applied to everyone or none.
Against an agent like this, who will have no specific dress code, whose real intentions we cannot know because we cannot read minds, how do you know when they are intending to act in a hostile fashion toward you directly?
You study the human nature which is common to all humans, you develop a system that gives you the ability to react quickly to the behaviors that can be observed at a distance, and you begin watching for the patterns as they manifest in innocuous circumstances to better recognize them in volatile ones.
What behaviors are you looking for? What body language? Which patterns?
You'll need to read the book to find out, and then re-read it again and again till it's all sunk in and becomes second nature, which is where I am working towards right now. It's a worthwhile and increasingly necessary effort, because with each terrorist attack we are reminded that even the pure civilian gets drawn into 4GW conflicts, The book will seem to state the obvious, yet when you start practicing the observations, you will realize there is much you've missed.
Read the book and stay Left of Bang.
Where I bought the book from on eBay.