What about The Spirit of the Liturgy by Joseph Cardinal Ratzinger? I would recommend it. It gives some interesting insights into the development of the liturgy, where we’ve come from and where we are now.
Or, and this is rather off topic, what about the Pope’s new interview-book Light of the World? I got hold of a copy as soon as it was published a few months ago, and while his opinions, of course, cannot be taken as official Church teaching as it is the result of a private interview, it is most interesting to get an insight into the mind of Pope Benedict in 2010!