I am quite fond of the Secret of the Golden Flower.
Jung became immersed in the study of medieval alchemy in order to provide a model for his own psychological theories, publishing ‘Psychology and Alchemy’ in 1944. His work is credited with directly leading to a revival of interest in this area of occult knowledge. He often emphasized the importance of the alchemist’s quest for the “philosopher’s stone”, which could transmute base metals into gold; a metaphor for the spiritual transformation of the self.
In alchemy, the philosopher’s stone resulted from the androgynous union of divine opposites (dark and light), and for Jung, like a cocoon and butterfly, was a symbol for the metamorphosis of higher self. Psychiatric analysis was a form of alchemy, he claimed, and each of the alchemist’s ingredients had a psychological equivalent. In this regard, iron was courageous and passionate, while tin was truthful and lofty. The element of Mercury, seen as the toxic, deceptive, transformative element which made the union of opposites possible, was by Jung’s definition, representative of the collective unconscious.