To a great extent, the plans and goals you must pursue are determined by the pawn structure; in this sense, a book like "pawn structure chess" by Soltis might be close to what you ask for. It foundamentally outlines the typical plans for both sides deriving from the most common pawn structures(and as a consequence openings), as well as the main pawn breaks avaiable for both players. It is surely a very instructive book, although i fear it might be a bit tough to understand for your current level.
General concepts / themes in openings?

Fundamental Chess Openings by Paul van der Sterren is probably what you need. It gives the overall plans for both sides and then a detailed explanation of the "why" behind each move. It is not just a listing of variation after variation without explanation like MCO. When you look it up on Amazon.com pay no attention to the fool who gave it a bad review saying that it's not in algebraic notation. Apparently he'd never seen figurine algebraic before or was drunk when he wrote the review.


DelCheMethod
You have it right--play to A, but if B or C present itself favorably, transpose into the other line. You shouldn't go into the opening with ideas like everything is worked out to move 30, you'll miss alot of opportunities over the board. Understanding how to safely slide from line A into line B, now that will gain you some full points and save some half points in your carreer. You're on the right track.
I buy into the argument of not memorizing line after line of opening variations for beginners. After all, I'm sure both me and my opponent are both out of book after move 3 or 4 lol.
However, I have read nuggets of wisdom from time to time of what you hope to accomplish with your opening. Like the importance of the light square bishop in the Ruy, creating the open c file in the najdorf, locking up the center and the pawn storm and first break in the KID, etc.
As a beginner, I think if I knew what my goals were with the opening I choose (beyond the GPS of develop pieces, get king to safety, dont move same piece more than once,etc) I could still develop toward those when we are out of book after the first few moves.
Does anyone know of a resource that list the general concepts of the various openings?