Paul wrote a couple of letters to the Thessalonians, people who'd been converted to Christianity only a few months before. (These are a couple of the earliest works in the New Testament.) Paul had been run out of town (Thessalonia) because there was a lot of opposition, mostly from Jews who were resentful of Paul's stealing members away to become Christians. Despite the protests, Paul managed to convert enough people to join the Christian church, which he hears about from Timothy who'd been there after him.
Anyway, so Paul writes to these new Christians saying hooray for following Jesus, and tells them they can expect a great reward when He comes again.
"For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord. Wherefore comfort one another with these words." -- I Thessalonians 4:16-18
Like a thief in the night, Paul says Christ will come again (5:2), which means they should be joyful, but also vigilant in following the teachings of the gospel and be good Christians all the time. Paul emphasizes the need to work hard, pray often and be thankful to God. In his second letter to the Thessalonians, Paul continues this theme, telling these people Jesus will come again to judge sinners, but more importantly, to save believers.
"But the Lord is faithful, who shall stablish you, and keep you from evil." -- II Thessalonians 3:3
Not much to comment on this book, since it's rather straight forward and nothing in it seems particularly surprising considering what I've read so far of Paul's letters. Well, except for the idea that Jesus/God will come again suddenly in the night. I don't know if I would have felt that was entirely something to be joyful about, if I were a Thessalonian. Anyone who sneaks up on me tends to scare me, which I'd say is pretty natural. And if it were Jesus, I'd want to be ready. I guess that's the point. If I feel I'm not ready, I should work on being ready all the time... Hmn... point taken.
**NEXT: I & II Timothy