Of Jung's works that I have read, this is one of the hardest to read. It is mainly about the different aspects of the opposites that are found in the unconscious, the archetypes that they correspond to, and how the conjunction of these takes part in the self during the process that he calls individuation. This might sound a bit woolly and mystical, but this is mainly as a side effect of his adoption of terms for psychological phenomena that also correspond to related phenomena from the worlds of alchemy and religion. He explains how the characteristics of such mystic experiences and beliefs can be explained by processes that happen in the mind, and how their origins can be explained rationally. So, what can at times seem like a work obsessed with the occult and mystic, is really a triumph of rationalism and science. The ideas that he proposes are compelling, and do a lot to explain to strangeness of the thoughts of people that lived hundreds or thousands of years ago. The readers opinion of the history of religion, magic, alchemy etc. will then change from it being a catalogue of folly and madness, to a catalogue of symptoms of healthy and ill psychological process that can be at least be partly understood.
However, the work does not seek to do away with mystery where it still exists, and the last part of the book makes clear that we do not understand the world and the mind as well as future scholars will. Jung's work is huge in its scope, and the reader must have read some of his related works such as the Archetypes and the Collective Unconscious, to fully appreciate quite how significant his thoughts are. As this is one of his harder books, that develops his simpler ideas further, it is probably a good idea to read some of his easier works first. ( )