Well, I just ordered my RRA lower to begin my build. This will be my first AR build and I've only shot 2 AR's total. Not really sure what I'm wanting to build it out to be, other than a bad a$$ rifle. I'm looking for a book that evaluates the different types of uppers, barrels, and parts in general. For example, I was considering an RRA EOP or UTE2 upper until some other members recommended the A4 as it is more versatile. This is exactly what I'm looking for. Something to say the pros and cons on different parts. Any recommendations?