Book

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 If rather outdated.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.

"Modern Chess Openings" (MCO) by Nick de Firmian

It is going to take FAR more than 1 book.

Ruben Fine's book is where to start with concepts. After that, figure out which ones make the most sense to you. Not which can you memorize the longest. Which makes the most actual sense to you. For one person, that may be the French, while for someone else, that might be 1...e5.

At that point, you need to invest in books (PLURAL) that ficus on your openings of choice. Many cover a single variation of an opening, like just the 6.Bg5 line of the Najdorf.

If this gives you an idea, I own over 20 books on the French and 4 books on the Petroff and 1.e4 e5 White does not play 2.Nf3 (the two openings I play against 1.e4 as Black).

A single book won't cut it. That would be like expecting to be a Certified Public Accountant after having aced a kindergarten math book.

Fundamental Chess openings by Paul van der Sterren

or

Understanding the Chess Openings by Sam Collins

"Modern Excelsior: Openings and their secrets".

Books on chess openings are an excellent tool for improving your game. I recently read material on the Italian game, greatly improving my understanding of strategy. Such resources help you look at the game differently, learn new variants and understand their strengths and weaknesses.