Something is essential if it is of the utmost importance: basic, indispensable, necessary.
Whether you sometimes disagree with a colleague or spouse, buy a car, or run an organization.
We are all negotiators.
It is back and forth communication designed to reach an agreement when you and the other party have some interests that are shared and others that are opposed.
Negotiation, in simple terms, is the basic means of getting what you want from others.
This book is just that, essential.
Something is essential if it is of the utmost importance: basic, indispensable, necessary