C. G. Jung 
Psychology and Alchemy [PDF ebook] 

Alchemy is central to Jung”s hypothesis of the collective unconscious. In this volume he begins with an outline of the process and aims of psychotherapy, and then moves on to work out the analogies between alchemy, Christian dogma and symbolism and his own understanding of the analytic process.

Introducing the basic concepts of alchemy, Jung reminds us of the dual nature of alchemy, comprising both the chemical process and a parallel mystical component. He also discusses the seemingly deliberate mystification of the alchemists. Finally, in using the alchemical process as providing insights into individuation, Jung emphasises the importance of alchemy in relating to us the transcendent nature of the psyche.