I agree that that's a helpful book, but be careful. It's good in that it points out what can work and what can't, and helps you put life experiences and ideas into a short essay. However, if you follow those guidelines you'll be creating a cookie-cutter essay that's similar to the other 4,000 essays the adcoms are reading. My advice would be (and I've also had interviewers tell me my ps stood out and they remembered parts of it) to just be yourself. Say what you want to say how you want to say it. Also, what really helped me was having an older MD where I work review it (several times). I figured he was representative of who would be reading it, so I wanted to make sure it worked. Good luck!