Outside of Gurdjieff’s own writings, there are a number of books available that introduce basic work concepts.

• A Simple Explanation of Work Ideas, by Maurice Nicoll. Short and to the point.
• Gurdjieff: A New Interpretation, by Wojciech Konrad Kulczyk PhD. Includes material from Gurdjieff’s own writings rather than relying too heavily on Ouspensky’s exposition.
• Gurdjieff: An Approach to His Ideas, by Michel Waldberg. Similarly includes material from Gurdjieff’s own writings.
• Towards Awakening: An Approach to the Teaching Left by Gurdjieff, by Jean Vaysse. Also short and to the point.
• Gurdjieff: The Key Concepts, by Sophia Wellbeloved. A reference manual of concepts. Includes some short biographies of Gurdjieff’s pupils, various groups that formed after Gurdjieff’s death, and the passive deviation from Gurdjieff’s teaching found in “New Work”.
• The Psychology of Man’s Possible Evolution, and
  The Cosmology of Man’s Possible Evolution, by P.D. Ouspensky, his particular exposition of Gurdjieff’s ideas.

Take these with a grain of salt. Learn what Gurdjieff taught from reading his own writings and working out your own understanding with guidance from a school and instructors rather than relying solely on another’s exposition of Gurdjieff’s teaching.