The Secrets of Consulting

is the title of an excellent book by Jerry Weinberg. As usual I'm going to quote from a few pages:
There's always a problem. It's always a people problem.
People want training that makes them better adapted to their present task, rather than training that makes them more adaptable to future tasks.
Consulting is not a test for the consultant, it's a service to the client.
Things are the way they are because they got that way.
Study for understanding, not for criticism.
Don't be afraid to cultivate your unconscious.
Cucumbers get more pickled than brine gets cucumbered.
The trick to earning trust is to avoid all tricks.
Never promise anything. Always keep your promise.