Sabbath School Lessons on Church Organization and/or Unity
Unity in Christ by Denis Fortin. Thirteen lessons on unity in this quarter.
Denis Fortin also wrote a companion book on the same topic, One in Christ: Biblical Concepts for a Doctrine of Church Unity in the Bible Bookshelf series, which is available in Kindle format.
Ellen White Notes for Unity in Christ, Fourth Quarter 2018, Pacific Press version. The “official” version of Ellen White comments on.
Also see the following:
Lesson 6: United in Diversity (lesson from 1999, 4th quarter)