The Mystical City of God is a book written in the 17th century by the Franciscan

Franciscan

Most Franciscans are members of Roman Catholic religious orders founded by Saint Francis of Assisi. Besides Roman Catholic communities, there are also Old Catholic, Anglican, Lutheran, ecumenical and Non-denominational Franciscan communities....

 nun, Venerable Mary of Jesus of Ágreda

Maria de Agreda

María Fernández Coronel y Arana, Abbess of Ágreda or, known in religion as Sor María de Jesús de Ágreda , also known as the Lady in Blue and the Blue Nun, was born, and died, in Ágreda, a town located in the province of Soria, Castile and León, Spain...

 or María de Ágreda

Maria de Agreda

María Fernández Coronel y Arana, Abbess of Ágreda or, known in religion as Sor María de Jesús de Ágreda , also known as the Lady in Blue and the Blue Nun, was born, and died, in Ágreda, a town located in the province of Soria, Castile and León, Spain...

.

According to María de Ágreda, the book was, to a considerable extent, dictated to her by the Blessed Virgin Mary

Blessed Virgin Mary (Roman Catholic)

Roman Catholic veneration of the Blessed Virgin Mary is based on Holy Scripture: In the fullness of time, God sent his son, born of a virgin. The mystery of the incarnation of the Son of God through Mary thus signifies her honour as Mother of God...

 herself as the title suggests; Also, the book is in part visions she received and tried to explain with her own expression. The book discusses revelations about the life of the Virgin Mary and the divine plan for creation and the salvation of souls.

María de Ágreda was declared Venerable soon after her death, but the book (which makes a number of somewhat unusual claims) has remained controversial within the Catholic Church and her process of beatification (started in 1673) has not been completed. The book has had an unusual history in the Church, in that it has been banned and restored a number of times.