I always found the Mystical City of God to be nearly impossible to read, but I think it was because I approached the work of Ven. Mary of Agreda more out of curiosity than devotion. It seems to me that I need a special grace to be able to read the work attentively. I'm only now beginning to read a few sections corresponding to the mysteries of the Rosary, and I sense Our Lady has granted me the grace to do so - perhaps through the intercession of Ven. Solanus Casey, who used to read the book while kneeling.