Dark Night of the Soul is an instructional religious book used mainly by the Roman Catholic Church. The premise of the book is that there are two "dark nights," or times of spiritual bleakness that a person must travel through in order to come to a completely mature and perfect union with God. It during these times of spiritual darkness that God refines and perfects the spirit of a man to be more like God himself. The original text was written in Spanish in the 16th century by St. John, a Roman Catholic priest and poet. Since the original Spanish text has been translated into English, there are numerous footnotes, which provide alternate translations or other references for particular passages.

Much of this book is based on a poem called "The Stanzas of the Soul." The complete text of this poem is included in the prologue of the book. Throughout his book, St. John refers to the poem as he writes about the different stages of love through which the soul must pass. In several places, St. John goes into much detail about the meaning of the imagery and symbolism of the poem, which at first glance appears to be about the love affair between two humans. After the poem is interpreted, however, one comes to understand that it is actually a poem about the love between God and the human soul.

St. John begins his work by describing the common failings and sins of beginning Christians. He does this so as to enlighten these beginners to the traps into which they may fall during their early days as Christians. These failings are introduced as they relate to each of the seven deadly sins as described by the Catholic Church. These sins are: pride, sloth, envy, avarice, luxury, gluttony and wrath. These failings aren't general failings, but failings as they directly relate to the Christian's life and their relationship with God.

After addressing these failings that can affect beginning Christians, St. John describes what he calls the dark night of the soul. Along with explaining what a person going through a dark night of the soul will feel and experience, St. John also gives three criteria to determine if a person is truly experiencing this dark night. These criteria are given because a person may experience feelings similar to those associated with a dark night but will not actually be going through a dark night. It is during this dark night of the soul that God works in the beginner's soul to purge the desire for sensual pleasures and helps the soul recognize and enjoy spiritual pleasures. The soul will reap additional benefits from a successful journey through this dark night such as humility, delight in peace, and a deeper reverence for God.

In the second book, St. John describes the dark night of the spirit. It is during this dark night that God works to bring the spirit into perfect union and love with God. In a similar way as in the first book, St. John describes how a soul traveling through this dark night will feel and what it will experience. He also details how it is essential for the soul to have successfully passed through the first dark night before it enters the second dark night. Also in the second book, St. John describes the ten steps of the ladder of love. This ladder leads the believer from his worldly bonds to a perfect, complete union with God. While climbing this ladder of love, the soul wears a disguise in the colors of white, green and purple to hide it from the devil, worldly pleasures and human nature.