Bosch's classic work - essential reading for undergrad theology - on the reading list of my theological college and university. Highly respected and very quotable if you are writing on the topic of mission in Christianity. Bosch describes the changes underlying mission in Christian history and especially the shifts in focus over the last century. He grapples with the question of "what is mission" both in the past and in our time. Bosch can be quite a tough read at times, it is easy to get bogged (Bosched?) down. My advice is to read selectively, focus on bits relevant to your ministry, and make use of a highlighter pen to mark the bits that grab you.
Whilst this took a while to get into, being a very academic text, it is actually a very interesting and useful read, especially for anybody studying mission. It looks at the church adapting to the culture of the time for success in God's mission and how this is relevant today.