Here are some of his notable writings:
* The Christian Endeavor Movement: Its History, Principles, and Methods (1886): This book outlines the principles and practices of the Christian Endeavor movement, which has been highly influential in shaping youth ministry and Christian outreach.
* Christian Endeavor World: He also founded and edited this influential magazine, which promoted the movement and provided resources and inspiration for Christian Endeavor societies.
* The Continent: A Journal of Religion and Life (1902-1927): Clark was a founding editor of this journal, which addressed a range of issues concerning faith, culture, and society.
* The Christian Endeavor Manual: This manual, written by Clark and several other contributors, provided practical guidance for Christian Endeavor societies.
* The Gospel of the Unseen King: Clark wrote numerous devotional works, including this classic book on the teachings of Jesus Christ.
* The Ideal Life: A Series of Sermons on Christian Character (1910)
* The Charm of the Cross (1912)
* The Holy Land of the Christian Faith (1916)
* The Religion of a Man of Today (1918)
* The World's Debt to Jesus (1924)
Clark's writings addressed a wide range of topics related to Christian faith, practice, and life. His legacy is largely tied to the enduring influence of the Christian Endeavor movement and his efforts to promote Christian values and action in society.