To further explore the Diego Rivera mural "Pan-American Unity," the City College of San Francisco's Diego Rivera Mural Project has a resource-filled website, including detailed images of the mural.
For more information about the Unitarian Universalist Association's conflict with the Boy Scouts of America you can visit this page: The UUA and the Boy Scouts of America. It includes copies of some of the letters that went back and forth between the two organizations. For up to date information, including a link to the 2016 Memorandum of Understanding between UUA and the Boy Scouts of America, please visit Religious Recognition Awards for UU Scouts.
Two books that explore liberal and liberating faiths and consider the role of Unitarian Universalism in these two movements are: A Reason for Hope: Liberation Theology Confronts A Liberal Faith by Fredric John Muir (Carmel, CA: Sunflower Ink, 1994) and Black Pioneers in a White Denomination by Mark Morrison-Reed (Boston: Skinner House, 1994).
Charles Howe's book The Larger Faith: A Short History of American Universalism (Boston: Skinner House, 1993) goes into more detail about early Universalist history including the story of the Gloucester Universalists suing for religious freedom.