St. Bonaventure had this to say about the Sacred Heart of Jesus (my translation):
“O ineffable beauty of the most high God, O purist splendor of eternal light! You are life that animates every life, light that illumines every light and keeps in eternal splendor the multi-formed luminaries that shine before your divine throne since the dawning of first light. O eternal and inaccessible, splendid and sweet flowing from the fount hidden from all mortak eyes! Your depth is endless, your height limitless, your breadth infinite, your purity undisturbed! From you gushes forth a river ‘that gives joy to the city of God’ (Psalm 45, 5), so that ‘in the midst of songs of the multitude in festivity’ (Psalm 41, 5) we can sing songs of praise, demonstrating with the witness of experience that ‘in you is the source of life and in your light we see light’ (Psalm 35, 10)” — Bonaventure, Opusc. 3, The wood of life, 29-30. 47: Opera omnia 8, 79
Beautiful words to try to describe the Heart of Jesus.
Pope Paul VI wrote a book on the Sacred Heart a number of years ago. It is in Italian, and I don’t think it is translated into English. I may post on it later today.