Bobby Fischer, when asked by a young boy how to improve the boy's chess, picked up a copy of "The Ideas Behind the Chess Openings," and advised the boy to "read this book. This book has everything. It's one of the best chess books, and can definitely help to improve your game." Whenever the legendary Fischer speaks of how to improve one's chess, I have just a word of advice: Listen.
In comparing the 1943 edition of this book to the more recent 1989/90 works, it appears that most of the revisions involve the Indian and Sicilian Defenses. For good reason: These are still the overwhelming choices of many an international grandmaster to 1 d4 and 1 e4, respectively. Other than that, the text is about the same. In fact, the more recent editions even have a few misspellings and caption-reference errors to boot.
Still, I think that's a trivial reason for chess players to shy away from this book. The title states the principle: As long as you understand the IDEAS of the openings you play, you will be ready for nearly any surprises--curve balls, if I may--that your opponent might have in store. It made sense in 1943; it still makes sense today.
I would recommend this book to those in the beginner-to-intermediate range. I believe that, once you understand the ideas of your preferred openings, then and only then should you begin to intensify your studies of opening theory. After that, I feel that everything will fall into place naturally.