So you want to buy an AR-15, huh?
taught me more about AR style rifles than any other single source I have read (and admittedly I was very ignorant about them). It is focused on buy, versus building, an AR, but it covered a lot of manufacturers, terms, and why/wherefores that I saw referenced in many posts, but didn't have the background to understand. It might be of use to you.
In the end, I bought an AK instead of an AR...
But to each his own.