One of my Christmas gifts was Card. Ratzinger's The Feast of Faith, a collection of essays and other writings on "approaches to a theology of the liturgy". I'm 100 pages into it (of 150), and it's full of excellent insights into the liturgy, full of observations which are timeless and timely. His praise and his criticisms of the liturgical reform which followed the Second Vatican Council are sometimes specific, sometimes general, but he continues to return to a necessity of continuity.
What I have read so far dealt with 1) the theological basis of prayer and liturgy, 2) the form of the Eucharistic celebration (is it a meal? a sacrifice? both? which first?), 3) the structure of liturgical celebration, and 4) what it means for some things to be mutable (changeable) or immutable (permanent) in the liturgy. The remaining four sections deal with liturgical music, the significance of Corpus Christi, orientation during liturgical prayer, and parish life 15 years after the Council.