Of course the difference is that the truly great Christians authors do not need a marketing department to keep their work before the public. Theological fads some and go. Who will be reading "Purpose Driven" in 100 years, or 10 for that matter?
And the truly great Christian authors point people their need for the Lord God in all aspects of their lives, not the need for their books, their sermons, their tapes, etc.
The title itself should cause us to wonder a little. Throughout all of scripture, God's people are being referred to as sheep. HE is the Great Shepherd and He talks about false shepherds. Sheep are a flocking animal. They are herded animals which are driven, but they follow. Jesus said, "Follow Me" a total of 17 times in scripture. I can't think of one time where He said He will drive, herd, push us to good works. Our lives aren't supposed to be "purpose" driven. Instead, we are to Follow Him because we love Him and want to be transformed by Him. There is a big difference. One leads to true peace with the Almighty God - through the blood of Christ Jesus, the other leads to works of our own or other taskmasters which lead to frustration and false hope and are wearisome (Ecclesiastes 12).