The answer is none. All true Christians need is the words of Jesus.
How many publications do people need to study to understand the law of Christ? Answer? Only one. All we need is the words of Jesus to be a Christian. The Bible says that Jesus is the path to eternal life. John 6:67-69.
Obedience to an organization or slave is not required.
Being a true Christian is simple and it's 'freedom'. No Watchtower literature is required. Why? Because extra doctrines would be adding to the words of Jesus. It would be running ahead of what God's son taught. The Bible says that 'anyone who runs ahead ... does not have God' 2 John 1: 9 - 11 NIV