FCO: Fundamental Chess Openings
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It gives very good explanations of the general themes, historical development, and early move orders of pretty much every opening you could want, although it does have a strong focus on mainlines.
For a beginner or intermediate, this book will give you enough theory to get started - your first half dozen moves with some deviations - but not enough to distract you from tactics and endgames. Coaches should love it. I'm sure they would tell you it's all you need until you are an expert.
