The Catechism of the Catholic Church has detailed explanations of the four Marian doctrines (including Assumption and Immaculate Conception) and is chock-full of both Biblical references and explanations of the Tradition.
This is a great source for anyone who seriously does want to understand the Faith, not just argue about it.
For those trying to understand about Mary, I suggest starting with 485-511, and 963-975.
Excellent suggestions!
BTTT on the Solemnity of the Assumption of the Blessed Virgin Mary, August 15, 2006!