"Seeking the World of the Marvelous" by Peter Ouspensky is a vivid account of Gurdjieff's teachings, as observed by the author during the period he worked with him, amidst difficult conditions of war and revolution. It is perhaps the most well-known text with Gurdjieff's ideas. Written with simplicity and sincerity, it also includes interesting anecdotes from Gurdjieff's interactions with his students.