To follow the bread crumbs back, here's the last one.
I should finish a little Bible study book on Philippians today. The format is quite annoying, very regimented, and it has been depressing how long it takes to write it--much harder for me than writing a normal book. But these little Bible studies might actually turn out to be the most appealing things I've written (I finished one on 1 Thessalonians about two months ago).
We'll see. They go through a book of Paul in six weeks of Bible studies, five days of lessons per week. Each lesson covers a little snippet of Philippians (anywhere from one verse to several). After a 50 word introduction, there's about 125 words of the central message of the snippet. Then there's about 125 words of commentary, followed by about 125 words of life reflection, followed by a short prayer. It would be good for a Bible study or Sunday School class or personal devotions.
Looking to Spain
If all we had were the book of Acts, we might get the impression that Paul's mission to non-Jews, to Gentiles, resulted from his continued rejection by Jews. We might think that he targeted Jews and Gentiles somewhat equally but just got a better reception from the Gentiles. However, Paul did not view his ministry in this way. Surely he was delighted when Jews came to believe that Jesus was their promised king, their messiah. But he did not see Jews as his calling. Paul understood himself as someone called to non-Jews. He was the apostle to the Gentiles, just as Peter was the apostle to the Jews (Gal. 2:8; Rom. 15:8-12, 16, 18). 
Paul also, as we said, saw himself as a traveling minister, not as someone called to a particular location until Christ might return. In Romans 15:20-21 he tells the Romans that he has made it a point not to preach the good news in places where someone else has already laid a Christian foundation. Perhaps Isaiah 52:15 was something like a "life verse" for him: "Those who have never been told of him shall see, and those who have never heard of him shall understand" (NRSV). 
He was thus a "church planter." He was not interested in staying in Rome for a few years the way he did at Corinth and then Ephesus and like he perhaps had done earlier at Tarsus. He seems rather to want to set up something like a "mission base" in Rome so that he could then launch into the unevangelized territory of Spain (15:24).
So Paul writes Romans from Corinth on his way to Jerusalem at the end of his so called third missionary journey. He writes more than any other reason to introduce himself to the already existing group of believers at Rome. He plans to go to Jerusalem before Pentecost (Acts 20:16) with a large delegation from the various churches he has planted (Acts 20:4). Together they were carrying a large offering for the needy among the Christians of Jerusalem (Romans 15:25-27; cf. Gal. 2:10). 
Paul is deeply interested in the Roman churches in their own right. He writes at the beginning of the letter that he had often wanted to come to them but that he had consistently been hindered from making such a trip time and time again (Rom. 1:13). He is hoping they might share together some mutual building up of each other's faith by his visit (1:11-12). He wants to talk "good news" with them, a good thing in itself (1:15).
But his primary destination is Spain rather than Rome. Rome is a very important stop along the way, perhaps both for personal reasons and for missional reasons. Surely Paul was looking forward to having an impact on the churches at Rome in part as a matter of personal satisfaction. But it is not his focus. His focus is to get to Spain, and he sees Rome as a fitting place through which to go on his way there. And just maybe he thinks the churches at Rome might contribute to that mission to Spain in the same way the Philippians seem to have supported Paul materially while he was in the other churches of Greece.
Romans is thus Paul's letter of introduction to the churches in Rome on his way to Spain. But as he introduces himself, he has at least two other tasks to accomplish as he writes. The one is to provide a defence of himself as a preacher of the gospel. Paul has enemies in the larger church. People talk about him, as Acts 21:21 indicates. So Paul writes Romans as the most systematic defense of his understanding of the gospel in any of his writings--at least in relation to the whole question of how the Gentiles can be saved without having to convert to Judaism.
He also apparently has some information about the church. He seems to know in some of the later chapters of the book that Rome has "stronger" and "weaker" Christians like Corinth did. So while he is introducing himself and his gospel, he also takes some time out to address some disunity he has heard about among the churches of Rome.
 The book of Acts, in its tendency to emphasize order, seems to emphasize Peter as the initiator of the Gentile mission in Acts 10 and at the "Jerusalem Council" of Acts 15. It is significant to recognize that Paul might not have told the story with the same emphasis. Indeed, he saw Peter at Antioch more as an obstacle to the Gentile mission (Gal. 2:11).
 The verse is in the famous "suffering servant" passage in Isaiah, which many early Christians seem to have read in relation to Christ's sufferings on the cross.
 It is intriguing that Acts never mentions this offering, even though it was an incredibly big deal for Paul (e.g., 2 Corinthians 8-9; 1 Corinthians 16:1-4; Romans 15:25-27). Acts gives us a lot of information that fits the framework of a delegation with an offering, but strangely does not mention the offering itself. We have to wonder whether the Jerusalem Church did not accept the offering or if the money ended up being used in some other way. Some would suggest Paul used the offering to pay for the purification rites of the men with a vow in Acts 21:24.