I've seen all of those spiritual/self-help books; a lot of them sell like hotcakes. (Does anyone actually BUY hotcakes, anymore? LOL!)
It's just a part of our culture. People are always looking for an answer, and it's easiest if someone just flat-out tells them what to do with their lives. (Hint to those floundering around in life: you'll find a good majority of answers in your Bible; read it!)
You might like "What Should I Do With My Life?" by Bo Bronson. It's a dozen or so examples of people actually making a difference in this world and living meaningful lives. (He also wrote "Bobos in Paradise" and has a good finger on the pulse of our culture today.)
I don't know about hotcakes, but I could go for a hot potato knish right now.
And those books are a waste of money. Folks should read the Bible instead. It's the best handbook on how to live a "purpose filled" life.