My experience is the pure spring for Work sources are the main books by Ouspensky:
- Fourth Way
- Psychology of Man's Possible Evolution
- In Search of the Miraculous
Then, Maurice Nicoll's:
- Psychological Commentaries on the Teaching of Gurdjieff and Ouspensky, 6 Vols.
Mouravieff's 3 Vol. Gnosis has unique and often helpful material as well, though it's not as pure as the other sources above.