What are some good books for chess openings? I am not new to chess, I have been playing for around 6 years. But I don't know much about chess openings technically. What are some of the very good books to to learn important opening moves?

Thank you in advance.
There are two excellent modern surveys of openings: Sam Collins' Understanding the Chess Openings and Paul van der Sterren's Fundamental Chess Openings. They both cover every major opening, demonstrating the main ideas and variations. I would start with Collins, since it is shorter and easier to read through in its entirety, but both are highly recommended.