The main reason that the Book of Ruth was written by Shmuel Hanavi

[edu]

There is a prohibition in the Torah that a Moabite can not enter into the congregation {or assembly} of the G-d.
The meaning is that he can convert but he, can not marry into the congregation. {For this halacha, converts are not considered "congregation" and therefore a Moabite convert can marry a convert from any other nation}.
However, traditional rabbinical interpretation of the prohibition is that it applies to Male Moabite converts and not to Female Moabite converts.

 Shmuel Hanavi wrote the book of Ruth, defending the halachic decision allowing Boaz to marry Ruth.
Now since King David was a descendant of Ruth, as the last verse of the book points out, it ends up that the prophet was essentially saying to us that King David comes from a valid lineage and is fit to be king.