I agree that Secrets of Consulting is a terrific book. I've always been amused by Weinberg's The Lone Ranger Fantasy: "When the clients don't show their appreciation, pretend that they're stunned by your performance--but never forget that it's your fantasy, not theirs."

In addition to Weinberg's writings, Steve Friedl's advice on consulting is terrific: http://www.unixwiz.net/techtips/be-consultant.html