I don't know how much help I can offer on this. Based on my reading, the idea seems to have taken rather a long time to develop and solidify, with scant explicit mention among the early Fathers.
If you want to get deep into things with this (and other Marian issues), might I recommend Fr. Juniper Carol's <em>Mariology</em>.
> Although some writings of Pope Leo the Great and Pope Gregory the Great would seem to exempt Mary from original sin, many more centuries of thought and prayer were required before the Church would realize that the Immaculate Conception was among the gifts God provided for His Mother. And still more centuries would elapse before the supreme magisterium would solemnly declare the doctrine of Mary’s freedom from original sin to be a revealed truth, i.e., contained in the original Deposit confided to the Apostles. (p. 19)
> In the present case, the first stage was the tranquil acceptance of the unique graces and privileges of Mary, which, as we now know, imply the Immaculate Conception. The early Christians accepted Mary’s singular position as Mother of God, as ever a virgin, as all-holy, as the New Eve. Thereby they implicitly accepted the Immaculate Conception, which is implied by the divine motherhood. During the first period of undisputed acceptance, the first liturgical evidences appear: feasts of the “Conception of St. Anne,” hymns, homilies. (p. 20)
> Theologically, we must face up to an evolution. From the extant philological data it does not seem that the personal sinlessness of Mary or her Immaculate Conception were explicitly taught as Catholic doctrine in the patristic West. (p. 163)
> The evidence does not justify us in concluding that the pre-Nicene East envisioned a rare or singular sanctity in the Mother of God, or believed formally that her conception was sinless. If it be insisted that the Immaculate Conception is a legitimate deduction from the patristic doctrine of the Second Eve, let it be remembered that it is deduction and that the deduction was not made before Nicaea. (p. 222)
I'm currently working through the first volume of a three-volume work on Mariology, which seems good thus far.
As for apparitions, there's this book, which I've not read, but has been lingering on my Amazon wishlist for a year or more.