"Influence" shows up on many reading lists, particularly in the business world, and it's been on mine for a while, if only to see what the fuss was all about.
The premise of the book is that although we learn a lot about psychology through research, there are people out there applying practical psychology on a professional basis. Cialdini terms them "compliance professionals," which is a bit of a euphemism compared to the words I would use. Cialdini himself has published quite a few papers in the field of compliance, and to write this book he combined his academic understanding of psychology with practical knowledge that he learned by infiltrating and going through the training programs in "compliance fields" such as car and real estate salesmen, advertisers, and marketers.
According to the book itself, there are two reasons for reading it. The first, and more legitimate reason in my mind, is self defence - unlike throwing a punch, psychological manipulation is so insidious precisely because you aren't aware it happens - it subverts your free will. So the only way to defend against it is education, and being aware of the techniques people might employ against you.
The second is that they are powerful tools if in the right hands; and of course, your hands are the right hands. I don't buy this argument, personally, because it doesn't generalise - you can't guarantee that everyone's idea of what is acceptable will be the same. Hitler thought he had noble ends as well, but I doubt many today would agree with him. It's a particularly relevant example because he was a master manipulator who convinced a nation that his interests were also theirs - using many of the same principles.
Even if, like me, you consider psychological tactics like these unethical, there is one good argument for reading this book, and that is to know thine enemy. Without knowledge of these things, you're exposed to weapons that you don't even realise exist. The analogy isn't a battle, because one person doesn't realise there is a battle. It's a nuclear attack - you can't defend against it and you don't know it happened till it's too late.
The book itself covers six different categories of human psychology - Reciprocation, Commitment and Consistency, Social Proof, Liking, Authority and Scarcity. Each section uses stories from his time in academia and in the field to illustrate various tactics and methods, as well as the research backing it up. They have names like 'reject then retreat,' 'foot in the door,' 'mirroring and matching,' 'being attractive,' etc. 'Foot in the door' for example is the old trick of asking for a small favour or commitment from someone that changes their identity in subtle ways, that then allows you to ask for bigger favours later. One of the studies illustrating this asked homeowners to display a gigantic billboard on their lawn promoting safe driving. Some of these homeowners had, two weeks ago, been asked to display a much smaller sign. Those homeowners accepted the giant sign (almost entirely obscuring the house) at a rate of 76% whereas those that hadn't been approached with the smaller sign refused 83% of the time. Just the act of displaying the small sign for a couple of weeks had changed their self perception to see themselves as responsible citizens who cared about safe driving.
This is pretty representative of most of the techniques - do something that intentionally taps into psychological heuristics in people's brains without them noticing, so that they comply with your goals. A section at the end of each chapter also provides the tools to mitigate these tactics being used against you. Some no more than trivial mental tricks or re-framing, but I won't spoil them here; it's a good book and you should read it for yourself!
Reading "Influence" was a tour through my first year psychology subjects. I enjoyed the book, but a lot of the examples were ones I was already familiar with, such as Milgrim's authority experiment, Zimbardo's prison, and Pavlov's dogs. The book was first written in 1984, and it seems that a lot of the backing research has been co-opted into introductory psych courses (probably to make them more interesting) and has filtered into the common consciousness through word of mouth, TV and the internet.
I'd recommend everyone read this, if only to avoid those moments of "why the fuck did I buy this?" that occur after a spontaneous purchase. If that's happened to you, you probably beat yourself up about the stupid impulse decision later when you got home, but a lot of the time those impulses aren't entirely your own fault. They're guided by people intentionally tapping into the primitive reactions and heuristics of your brain. Do yourself a favour and read it - education is the best defence.