The purpose would be to inform.
If the purpose of the authors writing is the persuade, it would look more like: Why You Should Negotiate With Others
If the purpose was the entertain, it could look something like: John Doe and the Negotiating Lizard
Since it’s a handbook teaching you how to do something, it is informing you on how to perform a certain task and how to do it properly.
So the answer is to inform.
