The books of I and II Thessalonians, which are in the New Testament, are both letters to a church that Paul the apostle helped establish in the city of Thessalonica.

First Thessalonians is agreed by biblical scholars to be written by Paul. The author of II Thessalonians, however, is still being debated about.
In II Thessalonians itself, and in secondary sources, there is evidence to support the theories that Paul wrote II Thessalonians as well as the first letter but also that someone wrote II Thessalonians in his name. First Thessalonians was written from Corinth in about 51 AD. If Paul actually wrote the second letter, it was written shortly after that. If he did not, then it is hard to decide when the book of II Thessalonians was written.

Since it is agreed that Paul wrote I Thessalonians, the text and style of II Thessalonians can be compared to it. If things such as vocabulary and style differ between the two, it shows that Paul may not have written II Thessalonians. An example of this is found in II Thessalonians 1:3 and again 2:13. The verses have the phrase, "we must always give thanks to God for you, brothers and sisters"
I found quite a few sites that believe that Paul did not write II Thessalonians but don't get me wrong there are also some who do believe the he did write this book but I will not make a decision because there because it is not clear from my readings of both books who is the actual writer is. I agree that this is an important part of Biblical history because it helps us date the writing of II Thessalonians.

In approximately 50 A.D. Paul had traveled to Philippi and then to Greece for the first time. During this time, he and two traveling companions, Timothy and Silas, they spread the word about Jesus to the Thessalonians. They formed a community of believers there and Paul was the founder of this new Christian community.

He, Timothy, and Silas exercised authority over the Thessalonians according to 1 Thessalonians 4:7. "we were able to impose our weight as apostles of Christ". The three stayed with the newly founded community for quite a while and it was not specifically stated as to how long they stayed all the book said was that "they stayed for a time to be".
The first part of the letter gives gratitude for the newfound faithfulness for the Thessalonians, who were idols that turned to God. The second part chapters 4 and 5 deals mainly with Paul urging the Thessalonians to continue to live their holy lives with full faith in Christ's second coming.

He assures them that if they continue their holy lives, those who die with full faith in Christ will be rise again upon Christ's return. Paul ends the letter by instructing them to watch for the Day of the Lord, which will come suddenly and unexpectedly, but only to those who refrain from evil.
These two books were some interesting reading, I can not say that these were the most interesting books that I have ever read but they were informative as to find out that some scholars believe II Thessalonians to be written by another other than Paul and here all this time I believed that Paul was the writer of both I and II Thessalonians. I will say that I am not disappointed but surprised that some of the scholars believe this book not be written by Paul but yet they do not name a specific writer who they believed wrote the book.