Theology
Theology as separate from Biblical Studies didn't occur to me until I started to look into the possibility of getting a degree from a(ny) seminary, and found that for the purpose of a BA the topics were treated as different fields of study. Before that I had always seen Biblical studies as a way to Theology and Theology as a way back to Biblical Studies, in my view they should go hand in hand, and I believe that this view is actually held by many Christians.
However, when you're looking for resources to study the Bible, Theological books are generally not the most helpful, and once you're well under way studying the Bible, the commentaries and historical books necessary for good exegesis will prove to be lacking in purely theological content (whether that is a good or a bad thing, you need to decide for yourself).
Theological books are generally considered to be somewhat difficult to read, thick books, with long sentences and incomprehensible concepts. A treatise on the Trinity will inevitably be hard to understand the first time around and will require a strong resolve to go back to the book, take the concepts back to the Bible, re-read and study the ideas of different theologians and then the reader will need to be able to put all the insights together into a practical belief system that is both in accordance with the Truth of scripture and can be applied in the daily life of the believer.
I think my first truly theological book was probably J.I.Packer's 'Knowing God', which I read while I was living in the basement of a chinese church in Taipei, in the winter of 1992. Whether it was my surroundings or the sense of separation from my family and friends, it was a book I thoroughly enjoyed reading at the time, and it gave me much to think about. It would become the beginning of a journey to and through different theological writers.
My second attempt at theological reading was thrown upon me by a good friend of mine, who enjoyed handing me books and then see how I would react to them. He handed me 'The Bondage of the Will' by Martin Luther (incidentally translated into English by J.I.Packer) which I read in the months leading up to my wedding. Great stuff to prepare you for marriage, I highly recommend it!!
The book that has been the one I have returned to more than any other Theological book over the years has been 'the Existence and Attributes of God' by Stephen Charnock, which I first read during the fall of 1998, and which proved at the time to be one of the most encouraging books for me to read. We were going through some serious financial difficulty, without support of a church and feeling somewhat alone and lonely, two young children and no way to see how the future was going to work out. Reading and meditating on the character of God (while doing odd jobs here and there to earn some money) turned out to be the best way to give me hope and encouragement. The book isn't light reading, and sometimes all I could do was read one paragraph and then sit back and let it all sink in, but it was the best book for me at that time, and since then it has often been the book to bring me closer to God, put me back on track and remind me of the incredible greatness of the God we serve. Very Highly Recommended.
Attributes of God _Arthur Pink
The Sovereignty of God _Arthur Pink
The Evangelical Dictionary of Theology _Walter Elwell
The Trinity _Gordon H Clark
The God Who is There _Francis Scheaffer
God's Empowering Presence _Gordon Fee
not purely theological books, but of interest to theologically minded people
The Jesus I Never Knew _Phillip Yancey
What's so Amazing About Grace _Phillip Yancey
Mere Christianity _C. S. Lewis
God in the Dock _C. S. Lewis
The Problem of Pain _C.S. Lewis
The Christian Mind _Harry Blamires
A Life of Jesus _Sushaku Endo
The Divine Conspiracy _Dallas Willard
The Climax of the Covenant _N.T. Wright
Who was Jesus _N.T.Wright
History of Christianity _Keneth Latourette
The Pilgrim's Progress _John Bunyan
Christ Plays in Ten Thousand Places _Eugene Peterson
Hoping to read before I get too old:
The Institutes of the Christian Religion _John Calvin
Karl Barth (any recommendations?)