I always suggest Chaos Theory as "Intro to Anarcho-capitalism." It's a very concise, readable, and powerful argument.

I agree that "Private Defense" is a little weaker, if only because I question the ultimate necessity of it. Personally I think that technologies like Bitcoin will lead to conditions where the modern Nation State as we understand it simply can't function properly, and it withers away. Under this scenario, nobody gets conquered because nobody can pay their military to do so anymore.

However, even if this doesn't take place we have to take the example of the US's failure to "conquer" Afghanistan, one of the poorest nations in the world, as some kind of example for now difficult it would be for any other nation-state to actually "conquer" the (former in this example) USA. I really think it's a non-problem.