You can read the Introduction online for free.
Gene Veith, Provost and Professor of English at Patrick Henry College, and author of the book God at Work: Your Christian Vocation in All of Life (inspired by Wingren's work) explains the impact that this book had on him:
Some years ago, a friend gave me a copy of Gustav Wingren's Luther on Vocation, saying, "You've got to read this." I put it on my stack of books to read, as others piled up on top of it. I thought I knew what the doctrine of vocation was. You do your work to the glory of God. What else is there to say? But when I finally opened Wingren's book, I found that Luther's doctrine of vocation is completely different than what I thought it was.
Vocation isn't so much about what I do, but about what God does through me. Vocation is nothing less than the theology of the Christian life. God calls us to live out our faith in the world, in the ordinary-seeming realms of the family, the workplace, and the culture. The purpose of every vocation is to love and serve our neighbors, whom God brings to us in our everyday callings. Wingren shows that vocation is also about God's presence in the world--which He providentially cares for through ordinary people, believers and non-believers alike--and about Christ's presence in our neighbor. Luther's exposition of vocation is imminently practical, offering a framework for how Christians can work out their problems in their various callings. It is the key to successful marriages and effective parenting. It also solves that much-vexed question for evangelicals today of how they are to interact with the culture.
Reading Wingren's book was one of those paradigm-shifting moments for me. It turned my life and how I see my life--its meaning, value, and purpose--upside down. It brought spiritual significance into the realm of the ordinary, where I live most of the time. I am convinced that recovering the Reformation doctrine of vocation--specifically, Luther's version--is a key not only in bringing Christianity back to the culture but bringing Christianity back into the everyday lives of contemporary Christians.