Mary is truly the Mother of God. (De fide)
This statement contains two truths:
1. Mary is truly a mother, that is, she contributed everything to the formation of the human nature of Christ, that every other mother contributes to the formation of her child.
2. Mary is truly the Mother of God, that is, she conceived and bore the Second Person of the Divinity, not indeed according to the Divine Nature, but according to the assumed human nature.
Catechism of the Catholic Church
Mary, Holy Mother by Scott Hahn
The Key to Understanding Mary by James Akin
Mother of God - Defender of Orthodoxy by Dwight Longenecker
The Mother of God by Tim Staples
Mary, Mother of God by Tim Staples