Over some 15 years, I must have given away the same number of copies of this book to my clients as well as to consultants who have worked with me on projects - and there is still a wealth of insight for both.
For the client, there are real life examples on how external consultants can help you build your business - or destroy what you already have. You cannot abdicate management responsibility, and you need to ensure that the consultant's objectives, authority, roles, deliverables and spend are clearly defined and regularly monitored. You also need to get the key people in your organisation fully briefed and engaged.
For the consultant, it is a good reminder that on the one hand you can help bring valuable insight because the people on the ground can be just too close to the problem, but on the other hand, there are likely to be employees (perhaps not always in senior positions, and perhaps not always listened to enough) who know what the real issues and possible resolutions are - so seek them out. It is also a good reminder that you are not infallible and what is simply a fee-earning project for you, could be a matter of survival for the client.
For both, the client is not always right - nor is the consultant. So work in partnership, listen, solicit honest debate, and once you have agreed on a course of action, act decisively.