de Ropp wrote most of his books during his Sonoma County years. Among his most influential books (concerning spiritual development) are: "The Master Game" and "Warrior's Way: The Challenging Life Games". The first of these stands as his report on what he had learned from his teachers and from the writings of similar figures, as well as more mainstream psychologists, psychiatrists, and researchers into fields such as religion and the spiritual life. The second is in part a sequential biography, and was written near the end of his life; a significant dimension of its content is his very personal evaluation of the characters and contributions of Gurdjieff, Ouspensky, Madame Ouspensky, John G. Bennett (another direct disciple of Gurdjieff), Gerald Heard, Aldous Huxley, Timothy Leary, Stephen Gaskin, Alan Watts, Carlos Castaneda, and other figures serving as teachers of those engaged in spiritual quests. He is critical of those he views as false gurus or merely pompous, and attempts a fair-handed assessment of those he deems verbose but limited, whilst yet expressing genuine gratitude for those whose efforts he believes have enriched human life.