If there is one book that you have on your shelf dealing with Mariology, this should definitely be the one. This book, written by one of the greatest Thomistic theologians of recent memory, covers every official teaching of the Catholic Church concerning The Blessed Virgin Mary. Lagrange looks at how the Church has interpreted and taught from Sacred Scripture and Tradition over the past 2000 years. Just the chapter on the Immaculate Conception alone is worth the price of admission. He even looks at Saint Thomas Aquinas specifically, and he points out his alternating positions on the Immaculate Conception, and why he had these changing theological opinions.
Many falsely think that Aquinas was completely against the dogma of the Immaculate Conception, yet upon closer examination of all of his writings, Lagrange, like many authentic Thomistic scholars conclude, "... that we must distinguish between three periods in the life of St. Thomas as to his teaching on this subject" Lagrange goes on to point out how Aquinas certainly began his theological career endorsing the Immaculate Conception when he wrote his commentary on Peter Lombard's Sentences, changed his opinion later when he wrote his Summa, and finally in the last years of his life possibly changed his opinion once again. He writes, "In the last period of his life, however, from 1272 until 1273, St. Thomas wrote a work that is certainly authentic. In a recent critical edition of this small work made by J. F. Rossi, c. M., we read: "For she [the Blessed Virgin] was most pure because she incurred the stain neither of original sin nor of mortal sin nor of venial sin." If it be so, then St. Thomas at the end of his life, after mature reflection, and in accordance with his devotion toward the Blessed Virgin, again affirmed what he had said in the first period of his life. We must note other passages indicative of this happy return to his first opinion." If you are looking for a book to explain all of the Catholic Church's teachings regarding Our Blessed Mother, then you cannot go wrong with this book. After you read it you will certainly be able to defend your faith much better in regards to this subject. It is published by TAN books and has been recently republished now with a new cover.