Secrets of Consulting: A Guide to Giving and Getting Advice Successfully by GeraldWeinberg
The best book I've read on influencing people. (Note the subtitle.) Weinberg took a hard look at the difference between giving advice (that's what they pay you for, right?) and handing out his opinion on every subject.
This book contained several AhHa!s that changed my life. The most important, so far, is a law that runs "Every time you solve your most urgent problem, you promote your second-most-urgent." For many people, this is simple common sense; it was news to me.
I think SecretsOfConsulting is useful to anybody who wants to be able to change somebody else's mind. For professional consultants, the book also has some brilliant advice on setting rates, finding clients, and other marketing necessities.
The most important AhHa! for me was the realization that one simply needs to accept and embrace irrational behavior from clients and other parties. All problems are fundamentally people problems. If logical behavior were abundant and sufficient to solve problems, no-one would need consultants. -- KrisJohnson
(See PeopleAreTheProblem)
Second that. QualitySoftwareManagement, volume 3, "CongruentAction", expands on this topic. It was a lifesaver for me, because I had thus far operated on an implicit model that people should always behave rationally. This wasn't working well and I couldn't understand why - I could well have gone crazy as a result. Now I have both redefined "rational" for myself, and learned to take the irrational, subterranean aspects of human behaviour into account. -- LaurentBossavit
Also see MoreSecretsOfConsulting by the same author.