When I first read this book, I found it very hard going. Possibly did not understand it. Perhaps even thought that it was badly written, as one is overwhelmed by the wealth of alchemical material. Jung's colossal intellect running miles ahead of me. Coming back, (after twenty years!) I find my opinion has changed.
Firstly, I will concede this is definitely not a book for the Jung beginner. It is one of deep scholarship. The fruit of over twenty years of study by Jung's to come to terms with alchemy.
The essential thesis is that the symbols and processes described in alchemical texts mirror those of the psyche at its deepest level. Understanding alchemy helps in understanding our psychological and spiritual development.
Jung then develops this with an analysis of a long series of dreams by a patient. The patient has since been revealed as Wolfgang Pauli, one of the pioneers of the Quantum Theory. In this Jung shows how the symbols are similar to those in alchemy. There follows a discussion of religious symbols in alchemy.
All of this when I first read it was hard going. My problem was a lack of understanding of alchemy. Having read around the subject a little further, I now find this work easier to understand. With some background knowledge, the book becomes very enlightening and enriching.
For background reading before tackling Psychology and Alchemy, I would particularly recommend Peter Marshall's Philosopher's Stone: A Quest for the Secrets of Alchemy
(which also has a handy glossary of alchemical terms) and Jay Ramsay's Alchemy: The Art of Transformation: The Path of the Soul
are particularly useful.