For the Novus Ordo (Ordinary Form) the useful book (literally “Liber Usualis”) is the Graduale Romanum published by the Benedictine monks at Solesmes in France. This book has all the ordinary and proper chants for Mass for the whole year. It is approved by the Holy See.
For the priest at the altar, there is not only the 3rd edition of the Missale Romanum, which has musical notation for the parts that can be sung, there is another book by the monks at Solesmes called the Ordo Missae in cantu, which provides all the chants the priests needs.
Read the whole article on Fr. Z’s Blog.