I think you're looking at the wrong category of book. FCO is about the best around at explaining openings, but you are right; it doesn't explain the why behind every move. Oh you could try Reuben Fine's old warhorse The Ideas Behind The Chess Openings (available in algebraic) or one of the multi volume opening series like Chess Opening Essentials 1-4 but I don't think you'd be much happier.
No, I think what you're looking for is an instructive anthology like Irving Chernev's Logical Chess Move by Move Every Move Explained. There is a whole list of books like that over at Coach Heisman's website, but Chernev is probably the easiest and most recommended.
http://danheisman.home.comcast.net/~danheisman/Events_Books/General_Book_Guide.htm#anthologies
As a beginner I first tried to understand the concepts of an opening without memorizing or studying openings.
I believe I have some understanding of these concepts but I'm missing the information on how to apply them based upon the context of my opponent.
I don't see the pro's and cons of the moves my opponent does in the opening and I seem to be just doing my part: trying to occupy/defend the centre, developing my pieces and castle.
I am at a point where I understand there is much more to openings than these concepts because the development of pieces need to be done with care and attention based upon your opponent his moves.
Because of this, I'm looking for a book that explains these concepts applied to the well known openings so I can start understanding why each move is done in a particular way.
I bought Fundamental Chess Openings because it got some good reviews however I feel it doesn't help me much because it doesn't go into details of each move. It does provide conclusions like "black is in slightly better position" after opening X or "white has the initiative" after opening Y but it does not explain why or how we came to this.
I don't just want to memorize these moves but I want to understand them as well. I personally feel that that's the only way to really master openings.
Can anybody recommend any books that will fit my needs?