Thanks for the link and the post with some of the Instructions from the Blessed Mother. Those direct instructions from the BVM that we can read at the end of many of the chapters in this truly magnificent work are among the most inspiring and informative ever in all of Christian literature!
I read the 4 volume set, about 800+ pages per volume, shortly after I returned to the Church when I was like 42. I was single, and had the time, and the quiet to think. I had hurt my lower back and was semi-bed ridden for like 7 days. I read the entire set. I could not put it down. What I found was that any questions I had in a chapter, were always answered by the Blessed Mother at the end of every chapter. It was uncanny.
A cousin that was the most pivotal person in my conversion, early on gave me a book to read, it was Novus Ordo (he didn't know tradition even existed, I later re-paid him by showing him tradition). He asked me what I thought of the book he gave me to read, and I told him it went in one ear and out the other, that I retained nothing. He said that was because the author was inept, that that does not happen with a good author.
Every word of Mystical City of God is profound and well retained. If I could only have one book in prison, this would be it, though it is 4 volumes, so not one book.