Not really a PU book but an absolute classic on the subject of persuasion.
Robert B Cialdini is an experimental social psychologist who spent 3 years studying the methods and techniques of what he calls "compliance professionals" , people who spend their lives trying to persuade and convince others ie salesmen, recruiters, advertisers etc.
He came up with with several new concepts in the field of psychology such as, reciprocation, social proof, authority and scarcity.
If you think this all sounds familiar, you'd be right as his observations have been widely copied, quoted and re-trodden by numerous pick-up artists, internet marketers, life coaches etc .
Definitely worth checking out and keeping a copy on your shelf, he is the original source on this topic.
Influence: The Psychology Of Persuasion by Robert B Cialdini is my recommendation for the day!