Book
Yes there is a lot. But you probably won't find that one book covers both explanations and variations. So begin with the explanation part.
Reuben Fine: The Ideas Behind the Chess Openings, is a classic. But there may be newer ones.
If you are able to read Danish, then Bent Larsen: Åbningsspillet i Skak, is to be recommended.
I agree with @Toldsted - Reuben Fine's "The Ideas Behind the Chess Openings" is a major classic. If rather outdated. I suppose it depends on what sort of book you want. Fine will give you the concepts but not a lot of concrete analysis. Sources like the old MCO or BCO or ECO will give you lines but not many concepts. As will opening databases. I always advise: first reflect and work out what sorts of middlegame position you do well at, then seek openings which take you to that sort of position.