I'm often asked the question, "Besides the Bible, what is your favorite book?" Pastor's are, by default, inveterate readers, so this is a very difficult question to answer.
For me, a "favorite" is not necessarily the best written or the most original, but the one I return to again and again. Lately, it has been Spirituality in an Age of Change: Rediscovering the Spirit of the Reformers by Alister McGrath. I have used this book as a devotional, for sermon and Bible study prep, and for sheer Gospel comfort. McGrath is a great historical theologian and prolific writer, and I have learned a lot from him, for which I am grateful.
In this little book, McGrath shows how the great Biblical themes of the Reformation can provide the spiritual depth necessary to meet the pressures and realities of the Christian life in any age, especially our present one. Those themes include:
The centrality of the cross;
The impact of God's grace when we understand its undeserved;
The affirmation of the laity as the "priesthood of all believers";
The dignity of ordinary vocations as service to God;
The reality of faith, doubt, and anxiety in the life of faith;
The importance of understanding "justification by faith"
If you are not familiar with the works of Martin Luther and John Calvin, this is as good as any place to start. This is not just a book for pastors or theologians. It's very accessible, and if you get a used copy (see link above), you will find out that the Reformers were down to earth, struggled like you and me, and could write comforting words that addressed sinner/saints with the Good News of Christ crucified and risen.