First Comes Love is a book about family, but not family in the way that the world thinks of it. Instead, it focuses on the Trinity and the Church. If we look at the Gospels, we can see that Jesus used familial terms a lot. He even went so far as to say that his family wasn't his mother, brothers, and sisters, but those who do the will of God. Dr. Hahn touches on this theme and more in this book.
The book starts off with a personal love story from Dr. Hahn. He talks about how he met his future wife Kimberly, and it was basically love at first sight. He compares the love he felt to the love Adam must have felt for Eve, and explains that man is made for woman and vice versa. He then continues his personal story of love begetting more love with the pregnancy of their first child. However, something went wrong with the labor, and she had to have an emergency C-Section. It was a success and both mom and baby were okay, but it definitely made Dr. Hahn look at his wife in a new and more beautiful way.
The rest of the book explains who God is and what the Church is. God is Family, Covenant, and Love. God also became man for us. He also explains Trinitarian theology in simple terms, and how the Catholic Church is like a home and family. My favorite section was actually the appendix, which discusses the Holy Spirit. St. Maximilian Kolbe said, The Holy Spirit is the uncreated Immaculate Conception." It was also interesting to read about the concept of bridal-maternity of the Holy Spirit. However, this doesn't mean we should refer to God as "mother" or that the Godhead has gender at all. "God created the human forms of physical gender and sexuality to be created reflections of the purely immaterial relations unique to each member of the Trinity." That's a lot to grasp, but makes sense the more you reflect on it. If you are looking to better understand the role of family in the Trinity and the Church, then this is the book for you.