An excellent book for this is Mastering the Chess Openings by IM John Watson. It comes in Volume 1, which covers e4 openings and Volume 2, which covers d4 openings.
Here's an excerpt on the Ruy Lopez:
"What's the point of 3. Bb5? One's first instinct is that it threatens 4. Bxc6 followed by 5. Nxe5, but Black's most popular answer 3... a6 shows that not to be the case, at least not immediately. Then of course White wants to castle quickly. But then why do most players use 3. Bb5 instead of the more aggresive-looking 3. Bc4, which hits Black's weak f7-pawn? The answer is that 3. Bb5 is a prophylactic move that works to squelch the opponent's opportunities..."
I am not a personal friend of the author. 
I'm wondering if I can get some help from this vast community here. I'm looking for a source (preferably online) that will help give me information about the strategies behind a good number of openings. I'm not looking for something that can give me "book" moves, I have MCO-14 and several databases for that. I'm looking for someplace to study the strategic ideas behind the specific openings.
For example, I know that with the Ruy Lopez, the idea is that the bishop on b5 is attacking the main defender of black's e pawn, and black needs to find another way of defending it, or risk losing the pawn, and then possibly the center. Those are the type of ideas Im looking for, but more in-depth. More of the why and not just the what. Does anyone know of a good source for such info?
Thanks,
Josh