I always tell people the best way to understand the opening phase, is actually a book on middle game play called "Reassess Your Chess" by Jeremy Silman.
It won't take the place of opening study, but it will give players an understanding of why opening moves are played. When people understand why moves are played, than can understand the underlying structure that various openings create.
But the book will not take the place of learning openings, just fill in the gaps. Most opening books whip out a few moves they expect players to memorize with little attention paid to why the first 2 to 5 moves are played.
In the long run, as you get more skilled, needing to know the guts of the first few moves becomes less of an issue from a playing perspective, but when your just learning chess or lower rated, it gives a lot of context to the moves.
Also, for me anyway, it's got me past the entire "Opening Principles" stage of playing chess. Opening Principles are very important, but it's used too often as a crutch for players to get out of the opening will not enough preparation. "Reassess Your Chess" doesn't trash opening principles, it just gives player the understanding that you can't just blindly move pieces in the opening merely based on a few guidelines that are only suitable for rank beginners.
Even though Reassess Your Chess is for middle game play, it always seemed to me to help me in the opening more. Getting out of the opening with a playable game is a huge deal.
In any event, I went through the book many years ago, it's on it's 4th or 5th edition by now.
i agree