The primary mission field for many Christians is your workplace. Evangelism is not the only reason God has placed you in your office but there is no higher calling—and no greater joy—than to see a co-worker meet Jesus. Many people work in an office environment with strict rules regarding evangelism and it is important that as a Christian you are above reproach in your workplace. But it is even more important that you are faithful to God. As you consider how to evangelize at work consider these things.Read the rest.
For a theology of work, I would certainly recommend Tim Keller's new book. John Piper also has a great chapter on this topic in Don't Waste Your Life.