I read St. Alphonsus Liguori's great book The Glories of Mary about a year ago. I found it very encouraging. It was fantastic to read about the power of Mary's intercessions on behalf of sinners. Some months later, I read that other great Marian work, True Devotion to Mary, by St. Louis De Montfort. Unfortunately, I did not find this book nearly so encouraging.
St. Louis De Montfort gives the impression that our devotion to Mary is useless unless it is perfectly motivated and springs from selfless adoration and love. One could easily feel reading his book that we can never be good enough for our Blessed Mother. In contrast, St. Alphonsus Liguori tells us that even the smallest devotions to Mary, like carrying a rosary or saying an Hail Mary once a day are enough to draw the mercy of the Blessed Virgin. By making small steps towards her, she will draw close to us. There is a much stronger sense of grace in The Glories of Mary.
I am sure these two holy men would not disagree in reality. Their two books are written for different purposes. There is a definite sense though, that True Devotion is about what we do for Mary, while The Glories of Mary is about what Mary does for us.
Read both books, but I would urge you to read The Glories of Mary before you read True Devotion to Mary.