You will find at http://www.philosophyoffreedom.com website a free online study course on the "Philosophy of Freedom". It is Rudolf Steiner's most important work, and the one that will endure the longest, because it describes his path to freedom.
When asked, "What will remain of your work in thousands of years Steiner replied: "Nothing but the Philosophy of Freedom. When asked which of his books he would most want to see rescued if catastrophe should come upon the world, Steiner replied: "The Philosophy of Freedom". When a student asked if he could attend Steiner's private esoteric training group, Steiner replied: "You don't need to! You have understood my Philosophy of Freedom!"