Moore says, pay attention to the dark, gloomy, melancholy aspects of your life and being. Do not avoid these parts of you and your life---they can be powerful transformative teachers leading to your search for wholeness and depth as a maturing and creative person.
Moore borrows the title of this book (and not much more) from St. John of the Cross, a sixteenth century Spanish mystic and poet Carmelite monk. Although Moore only quotes from St. John of the Cross 2-3 times and only short snippets at that, he carries the dark night imagery poetically through the entire book. I think in order to appeal to a wider audience, Moore borrows much more from Greek mythology, Carl Jung and James Hillman to present the widely experienced concept of Dark Nights of the Soul and how to journey through them. Moore, as an expert on soul care and as a therapist, presents dark nights as periods of transformation---usually through unexpected struggles of all varieties (spiritual disenchantment, emotional, mental or physical illness, creative emptiness, aging, relational loss, existential angst, etc). In this way, Moore spends a great deal of time normalizing dark nights as essential aspect of a maturing life convincing the reader not to run away from them, but entering the transformation of the dark nights more fully through non-analytical means: Imagery, poetry, creative expression, eccentricity, etc.
While there are many great quotes, here are a few key concepts:
"For here lies another paradoxical secret of the soul: That which seems to have twisted your life or personality for the worst is the very thing that will heal you and give you meaning" (p. 256).
"...[T]he end result is not a final victory nor an end to suffering. It is a moral development, the result of an initiation in which the mysteries of life stamp themselves into you more deeply, not necessarily making life easier or happier, but allowing it to take place more intensely. You are more fully who you are. You engage life more energetically and in that engagement discover a level of meaning that dissolves any discontent you may have" (p. 303).
"Whatever you say about the nature of things has its opposite, and so you are always turned back into the questions you ask, always faced with mystery. The same is true of the dark nights of the soul. They are full of contradictions, and the main paradox is that as much as they seem to plague you, they are your salvation. They can heal in a way nothing else can. They can erase the false logic by which you have lived your life" (p. 306).