Beginning on December 17th, as the final phase of preparation for Christmas, the Church recites or chants the O Antiphons preceding the Magnificat during Vespers of the Liturgy of the Hours. The O Antiphons express the Church's longing and expectation for the Messiah, her startled wonderment at the fullness of grace which the Christ-Child is about to bestow on the world. Their theme is the majesty of the Savior, His wisdom, His faithfulness and sanctity, His justice and mercy, His covenant with His chosen people, who in their ingratitude broke faith with Him. They are concerned with His power and love as King and Redeemer of the world, His relation to every soul as Emmanuel, God-with-us. (source: With Christ Through the Year by Bernard Strasser)
There also is a nice meditation and explanation of the O Antiphons over at the CERC by Fr. Roger J. Landry. You can find it here.
Lastly, the book pictured above, The O Antiphons: Poetry, Reflection, and Song to Being the Season of Advent, is from Grace Episcopal Church, but much of its content is gleaned from the aforementioned Fr. William Saunders at the Catholic Education Resource Center! It's a really beautiful PDF that you can download for your private prayer. It's only 18 pages and that's including the full hymns -- an easy devotion nightly! Enjoy!