Know your audience.
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Also : https://www.amazon.co.uk/Little-Red-Book-Selling-Principles/dp/1885167601
This book addresses the questions you're having directly. You might find it quite useful:
https://www.amazon.ca/Little-Red-Book-Selling-Principles/dp/1885167601
The Little Red Book of Selling by Jeffrey Gitomer. It's good.
http://www.amazon.com/Little-Red-Book-Selling-Principles/dp/1885167601
Here's another one http://www.amazon.com/Little-Red-Book-Selling-Principles/dp/1885167601