Lets start with the free stuff! Give these websites a try:
Apparitions and Shrines of the Blessed Virgin Mary (that have been approved by legitimate Church authority), and
Theotokos Books' page, Nine Major Approved Apparitions - click on each for good summaries
I would also invite you to scroll back through my blog posts over the past few days for information on how the Church approves Marian apparitions and my summaries of apparitions.
Now, if you're in the mood to fork out some dough, I recommend:
1) The Catholic Book Publishing Company's Dictionary
(Revised, Expanded Edition)
2) Donal Anthony Foley's Marian Apparitions, the Bible, and the Modern World