I would also like to recogmend "The Art of Deception" by Kevin Mitnick...this book is the Bible of social engineering. Mitnick goes indepth on a number of social engineering exploits and also gives suggestions on how to harden your network against these attacks.
I had downloaded it from the internet a while ago.
I found it a piece of crap. I've never searched for info about social engineering and everything what's in it I knew by yourself. There's no such thing as "learning social engineering", it is something you are either capable of or not. If you read one 3 page text somewhere on the internet about social engineering you know everything there is to learn about social engineering...the rest comes from your own brain.