It's one of the best books on Mary that I've ever read. Drawing on Scripture, the Fathers, the Lives of the Saints, and writings of the Popes, Father Manelli provides a concise defense for the claim that Marian devotion is absolutely necessary for our sanctification.
He also sheds light on the Marian titles of Mediatrix, Co-Redemptrix, Gate of Heaven, etc. As Saint Irenaeus said in the second century: "Mary became the cause of salvation for herself and for the whole human race" (Adversus Haereses 3:22).
You can purchase Father Manelli's Devotion to Our Lady here.