After completing the questline for Apule Vire, you acquire Aron Excavations Office as a house. If you upgrade the house completely (by talking to Vincent Helvern who is right next to the wooden ramp leading to the office and spending a small sum of gold 3 times), a bookshelf on the second floor contains this skill book.
Content[]
Trader's Manifesto
The game of trading is as wild as the seas and as different each day as the dice each roll.
The first rule when dealing with these southerners is never let them have the final word. Bargain, barter, deliberate and argue all you want, but at the end of the day, you have to add your terms, and agree to them.
The second rule is hospitality. At the bargaining table, a flagon of mead is worth more than all of the water in the ocean. If they think you are their friend, you will always benefit. If you think they are your friends, you will always suffer.
The third rule is knowledge. These southerners pride themselves on their libraries and magic, yet they lose their fortunes and spurn their wealth. If you know what they know, you are an even match. If you know what they don't, you are their superior.
This is the way of the Varani, and you will always find the proper deal if you follow this advice.