Ruth and Boaz's union is not a Levirate Marriage, but Levirate Marriages are alluded to many times and share a common theme to Boaz's redemption of Ruth. The union, at a great personal cost, establishes an heir for the deceased and cares for a widow who due to her Moavite ancestry is deemed commonly as unsuitable. 

Courtesy of the Virtual Beit Midrash, Yeshivat Har Etzion