1. Modern Chess Openings, 15th edition by Firmian
2. Here in Chess.com: highlight "Learn" above, then click Book Openings
Books on the openings
Sort:
MCO is a good reference manual, but it doesn't teach you the ideas behind the openings. Gosh, theres a lot of good books out there on different openings. Just pick some openings you like and buy the starting out book on it by everyman chess. Then you can memorize theory while having an idea about why the moves are played.
There are a gazillion out there- John Watson's 'Mastering the Chess Openings' vol 1 and 2 are good and, Everyman Chess 'Winning Chess Openings' are just a couple I have and are good
IrishMike 
Avoid play-by-route books. I like the classics: Reubin Fine's The Ideas Behind the Openings ("In the Latvian, if white prevents black's P-Q4, he has an easy game", "all Philidor traps are easily avoidable if black develops normally" etc.)and Chernev's Winning Chess Traps go well together. If you like tomes, and can find it, Horowitz's Chess Openings: Theory and Practice, although less complete than the latest MCO, (lousy on aforementioned Philidor) has invaluable Idea Variations and Practical Variations. I prefer older books as they are cheaper.
Pretty much any opening encyclopedia will do you well initially. Once you find a particular opening you are interested in, you can buy specialty books all about that opening that usually have better analysis than is contained in the encyclopedia. Because so many openings are covered by an encyclopedia, they have to limit the space devoted to analysis or else a single book could easily expand to multiple volumes costing hundreds of dollars and few people would buy it.
You should also familiarize yourself with the work of Fred Reinfeld. Most of his books are older, and you may have to scrounge the used book stores in your areas to find many of them, but they are indispensable to a beginner because his writing is so easy to read, even when he is discussing intermediate and advanced concepts.
Hello:
Looking for two, can't be without, books on openings and the theory.
Thanks!