Larry, I don't believe there is a single source of writing about the nature of the unconscious per se. There is an article called something like, "Unconscious mental activity in hypnosis - psychoanalytic implications" in Vol. 3 of the Collected Papers. You should hit the indexes of the Collected Papers (esp. vol 3.) and Clinical Casebook, and The Answer Within. You may find sufficient coverage that way. Also, be alert to the date of publication. Erickson's views evolved a bit over time and early writings did not always reflect the positions he took at the end of his career.