Parker was a prominent abolitionist and Transcendalist Unitarian Minister, writing extensively against slavery. His sermons and speeches would inspire later speakers, including Abraham Lincoln and Dr. Martin Luther King, Jr. 

Titles include: The Function and Place of Conscience, The State of the Nation, A Sermon of Slavery, The Boston Kidnapping, The Nebraska Question, The New Crime Against Humanity, The Laws of God and the Statutes of Men, Sermon on the Dangers Which Threaten the Rights of Men in America, The Great Battle Between Slavery and Freedom, A New Lesson for the Day, The Present Aspect of Slavery in America, The Relation of Slavery to a Republican Form of Government, The Effect of Slavery on the American People, John Brown's Expedition Reviewed in a Letter... to Francis Jackson.