Ruth is another of those stories with many parts. I want to talk about only one.
When you get to the end of the story, Ruth is the great-grandmother of King David. This means that King David himself would not, by the strictest Jewish law, be considered Jewish, since it is the mother's line that determines that.
That is the major point of the story for the people who first read it. Be careful who you exclude, because the greatest among you might not have come from the purest line.
The book was written at a time when the nation of Israel was debating inclusion and exclusion and purity and sin.
The point of the story was that it is always better to include (even the hated Moabites) then to exclude.