If you are not already aware of his work I recommend three books: Called to Communion, a short little book on the nature of the Church; The Spirit of the Liturgy, a biblical theological approach to the significance of liturgy; and Introduction to Christianity, his classic overview of Christian theology.
Two of the major themes of his work has been the need to do biblical theology and the importance of the Eucharistic celebration.
Another theme he has sounded again and again is the dangers of relativism. When asked a few years ago about the biggest problems facing the Church's mission to evangelize, he said the following:
I would say that today relativism predominates. It seems that whoever is not a relativist is someone who is intolerant. To think that one can understand the essential truth is already seen as something intolerant. However, in reality this exclusion of truth is a type of very grave intolerance and reduces essential things of human life to subjectivism. In this way, in essential things we no longer have a common view. Each one can and should decide as he can. So we lose the ethical foundations of our common life. Christ is totally different from all the founders of other religions, and he cannot be reduced to a Buddha, a Socrates or a Confucius. He is really the bridge between heaven and earth, the light of truth who has appeared to us.