There are thirteen letters from Paul hand which have been preserved by God for us (Hebrews not one of them) and there is evidence that he wrote many others which have not been preserved. 1 Corinthians 5:9 They are grouped in the Bible listing the longest first. The letters all deal with many different topics and problems however they can be broken down into three types of letters.

Doctrinal Epistles
Last Things
First and Second Thessalonians A.D. 50-51
These letters deal with the doctrine of the last things what theologians call this the doctrine of Eschatology. Paul deals with the Second Coming of Christ and how it affects the New Testament believers. Some had stopped working to wait for the Lord and they had to be encouraged not to neglect their daily work. Paul tells them what will happen to believers who die before Christ comes again. Paul also encouraged them in their persecution. 

Salvation
First and Second Corinthians, Galatians, and Romans A.D. 55-58
These letters look at different aspects of the doctrine of salvation what theologians call the doctrine of Soteriology. The Corinthian letters sets out the doctrine of salvation to many different areas of Church life. It covered such areas as personal relationships, leadership, spiritual gifts, immorality, ordinance of the Lord's Supper. And in his second letter Paul defends his apostolic office and biblical authority. Romans and Galatians set forth the doctrine of salvation in justification. Paul clearly set forth the fact that we are saved by grace and not by works. Martin Luther's life was changed reading these books.

Prison Epistles
Philippians, Colossians, Philemon, and Ephesians A.D. 60-62
These are called the Prison Epistles, because they were written from Prison. Paul was in prison in Rome and communicated with the Churches from his prison cell. Acts 28:30,31 These are more personal and intimate than the other letters of Paul. The doctrine of Christ is applied to many areas of life including the family, work, and the Church. The person and work of Christ is magnified like no other books. Theologians call this the doctrine of Christology. 

Pastoral Epistles
First and Second Timothy, and Titus A.D. 63-67
Pastoral Epistles deal with pastoral concerns of leadership in the local Church what theologians call the doctrine of Ecclesiology. Paul gives pastoral instruction to Timothy and Titus whom he has left as elders in the churches which he nurtured. It deals with officers, administration and other Church government matters.

General Epistles
James, 1 and 2 Peter, 1, 2 and 3 John, Jude
The General or Catholic Epistles describe the seven New Testament epistles consisting of James, 1 and 2 Peter, 1, 2 and 3 John, Jude. Hebrews is not a part of this group but stands by itself. These epistles are generally treated as a group distinct from the Pauline Epistles, Hebrews and Revelation.

They are called Catholic Epistles because of the destination of the letters. The word Catholic means 'universal'. These letters unlike the Pauline epistles, are not addressed to specific churches or individuals but to the universal Church. They have also been called General Epistles. There are two exceptions to this rule. 2 and 3 John are addressed to the elect lady and to Gaius. But these letters have all been accepted traditionally as the Catholic or General Epistles.

The Problem Of Suffering
James and 1 Peter
Throughout the first century the Church experienced many periods of hardship and persecution. These letters deal with the universally perplexing problem of suffering. They seek to address such questions as why do Christians suffer? What are the purposes of suffering? It was always distressing for the believers. They were perplexed by the purpose of all these experiences. 

The Problem Of False Teachers
2 Peter, 1, 2 and 3 John, Jude
The last five New Testament Epistles, are 2 Peter, Jude, 1, 2, and 3 John. Three of them are only one chapter. These letters deal with false teaching found in the early heresy of Gnosticism. This view held that you were saved by your knowledge or by receiving visions or mysteries. In 3 John Diotrephes is singled out as a dictator in the Church. The false teachers are shown to be immoral in character as well as heretical in doctrine showing how the two always go together.

The Supremacy of Christ
Hebrews
This book has been categorised under the General Epistles but it is a unique book. The author is not known and yet it has some of the greatest declarations of the majesty of Christ found in any New Testament book. It seeks to show the supremacy of Christ to the entire Old Testament sacrificial system and reveal that Christ is the fulfillment of all of its types. Christ is the final and great mediator who is superior to all of the Old Testament mediators. It becomes clear that the New Testament Christian could not continue under the old Jewish system now that the great High-priest Jesus Christ had come. There can be no turning back Hebrews says.