What to read before going to seminary? Here's my proposal for a list of vital theological works. I've not gone into confessional differences; you should know enough to read your Calvin, Luther, Wesley, or Aquinas if you come out of those respective traditions. I've also chosen works that are fairly accessible, and have been seminal for modern theology.
Other teachers will have different lists. This one is mine, reflecting my own education and interests.
1. The Confessions by Saint Augustine. Look for all the theology he incorporates into this hugely influential work, which is also the first autobiography ever written. Read it twice. Also, The City of God has become very important recently, but this is a longer work.
2. After Virtue by Alasdair MacIntyre. A classic work that points to the end of modernity as seen in its broken ethical systems and vocabulary. Very influential on a range of theologians.
3. Dogmatics in Outline by Karl Barth. A dense work, but it summarizes much of what he was about, and why he is the most significant and influential theologian since the Reformation.
4. Theology and Social Theory by John Milbank. Another dense and difficult work (Milbank can be notoriously impenetrable), but it has been very influential in his critique of modernity and its violence. You will see the influence of MacIntyre and Saint Augustine on Milbank.
5. Rowan Williams. The keenest theological mind in English today. The former Archbishop of Canterbury (retired, now the head of Magdalene College, Cambridge) is suffused with sacramentality, theological insight, and liturgical sensibilities. Anything by Williams will be good.
6. Stanley Hauerwas. The keenest American ethicist and theologian. There is a Reader of his writings, or The Peaceable Kingdom is quite seminal. He has been influenced by MacIntyre, is a persuasive pacifist, and is part of a postmodern turn to the church and its practices.
7. Sarah Coakley is a Professor of Divinity at Cambridge, an Anglican priest, and a feminist. She is liturgical and sacramental, and writes about gender and sexuality in deeply thoughtful ways. She is working on a systematic theology at this very moment. Anything by her will give you a sense of contemporary Anglican theology.
8. Follow Ben Myers' blog at http://www.faith-theology.com for keen observations on theology and academics today (along with some wit).
9. For other suggestions, see the The 3 Books Shelf series at The Theology Studio.