Man and his symbols - C. G. Jung et al.

Published shortly after Jung’s death, meant to be an introductory text to Jung’s ideas. 5+1 chapters, the first written by Jung and the others by close collaborators. It is introductory in that it does not contain overly technical terms, but it is not an easy read. Quite steep at times. link

The fundamental insight might be summed up as follows. Some animals are capable, in some sense, of abstract thinking. Take the ape who was taught sign language, take elephants. As humans, we must have gone through this stage, and gradually got to our degree of consciousness; the remnants of (for example) whatever the ape has in its head is in our heads still¹.
These remnants basically build up the unconscious. They are by construction common and dormant in all humans, so it must be possible to find them expressed in all human cultures; “underdeveloped” cultures must do it more visibly than our modern society. Several examples are listed in the book.
The unconscious is constantly bullied by logical thought, but it’s still there, and needs to be cared for. Just like a scratch left unattended might develop in a deadly disease, calls from the unconscious constantly suppressed or ignored might develop in illness of the mind. Like scratches, most often they don’t, but they can be nuisances, and they’re avoidable.
Mild discomfort of the unconscious is expressed through dreams; not all dreams are expressions of the unconscious. It is important to understand dreams; it is healthy to understand their message. Expressions of the unconscious come from the common ground upon which all human minds are built; these expressions are coded in universal symbols. It must be possible to find common dreams among all human cultures. The authors bring data to support this hypothesis.

This is an incredibly interesting read.