This is one of The great books about the Sacred Heart of Jesus. It is the story of the life of St. Gertrude the Great - her title alone tells much - and how Jesus revealed Himself to her -- His great love for her and for each and every soul, His burning desire for the salvation of souls, His sorrow, and even agony, at the loss of souls, and finally, His great need for our cooperation with Him by our practicing reparation for their salvation, and also by our practicing reparation to Him for the sorrow and pain that sin causes Him. Reparation is explained in the book. It is something that anyone can do, and does not require much time.
Examples are daily consecration of oneself to Jesus in His most Sacred Heart (a morning prayer), other special prayers and devotions, and most importantly, the offering of our daily life, our words, actions and especially our sacrifices - even if very, very small, such as denying oneself something special or putting it off until we have finished our work for the day, saying an extra prayer during the day, etc.; practicing God's divine economy of time - what we are here to do, and what we are not - and much more.
The children at Fatima also received in particular the message from Mary about making sacrifices in our daily lives for the salvation of souls. I remember reading that at least one of the children was surprised at how a very small sacrifice could result in the salvation of a soul, or souls.
When I first discovered Amazon, I sold my copy of this book. I have come to realize that my life's devotion is to Jesus, especially in His Sacred Heart, and also to Mary in her Sorrowful and Immaculate Heart. I feel that I literally can't live without the book and the advice and help it contains. If you have this devotion, you understand. Jesus also shows His great gentleness, compassion and love for St. Gertrude, and for every soul. A spiritual treasure.