What is a good book to learn about openings?

The priority for a beginner should not be to go deep into one variation, but to get an overview of the various openings. The overview is what you need when you go to create a serious repertoire, something you shouldn't do as a beginner but more like when you're around 1800. The FCO book recommended is good for this purpose.

I'd suggest actually 2 books by Vincent Moret (also featured in RussBells link):
My First Chess Opening Repertoire for White
My First Chess Opening Repertoire for Black
Those cost around 8 (€,$,£) bucks each and are well worth the money. It's not an overview over a ton of openings but it explains very well the ideas and aims of the openings it deals with (facing counterplay as well).

Stephen King's The Shining

You could also try Reuben Fine’s The Ideas Behind the Chess Openings and Fed Reinfeld’s Complete Chess Course. These are old but they continue to stand up.