By Gerald Weinberg, Dorset House, 1985, 0-932633-01-3 A short Gerry Weinberg book, which I find useful for the three laws he presents:

[p5] The First Law of Consulting: In spite of what your client may tell you, there’s always a problem.

The Second Law of Consulting: No matter how it looks at first, it’s always a people problem.

The Third Law of Consulting: Never forget they’re paying you by the hour, not by the solution.