If you want to read a good book with your girlfriend, choose one that isn’t boring but interesting and which goes deeper into Catholic theology. Read ‘Modern Saints’ by Ann Ball, it is very good and shows alot of Marian Saints. There are also the biographies of the Cure of Ars and Saint Bernadette Soubirous by Francis Trochu which are VERY good. Also highly recommended is Story of a Soul by Saint Therese. Its so heartwarming!
I suggest reading books about the Blessed Mother as She being the Mother of Jesus can do anything and can convert even the most hardened of sinners by Her love and mercy. There is True Devotion to Mary if you want a good book about Her which is a light read (I might say read the Glories of Mary but it isn’t very light. However, there are several stories in it showing the love of Mary for us). There is Mother of the Saviour by Garrigou Lagrange and although it is more theological and acedemic, it is still a very light, thoughtful, refreshing read. However, a very good book about Mary which I truly recommend is Miraculpous Images of Our Lady by Joan Carroll Cruz. Really, any book by her should be read but this being the Blessed Virgin, I bought it first! And there is also Secular Saints.
If you want a more spiritual book, read Introduction to the devout life by St. Francis de Sales. Its simple and to the point AND it was done by a Marian Saint.
Finally, I really, really, REALLY recommend yoou buy and read Fatima in Lucia’s own words, the accounts of Sister Lucy on the events of Fatima. Its the first spiritual book I ever read and the book which made me love the Blessed Virgin Mary.