It’s so good that she wants to become a Catholic . Books which I found good when returning to the Church were Catholicism for Dummies, Divine Mercy on my Soul (St. Faustina’s Diary) and a book called Catholic Devotions.
Catholic Devotions has instructions on how to say the rosary, prayers for different needs, it has about asking the Holy Spirit to help when you read the Bible etc, it’s really helpful and opens up your mind to the different types of worship.
Or you could get her a book on Saints, or the Marian apparitions. The Marian apparitions will probably help to strengthen her faith even more.
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