Once you reach Skyhold, you need to go to the War Table and complete the "Power for a Price" mission. Once you receive notification that the mission is completed, your reward will be a new vendor who sells these books. Make sure you have enough gold though to buy at least 5 at a time. The trick is to NOT close the vendor window and just keep buying and selling back the books. Do this about 20 times and you'll max out your influence to level 20. Have Fun!