This is so hard to say “must reads” as the depth of Catholic writing is so large, broad, and diverse. Additionally, each Catholic’s needs can be equally broad and diverse. However, these are some that I think hit different “needs” in no particular order:
The Delorous Passion of Our Lord Jesus Christ: Puts greater detail on the Passion of Jesus to better allow us to enter in the Passion. For me, just the description of the Agony of the Garden makes this book life-changing.
Why Do Catholics Do That?: A Guide to the Teachings and Practices of the Catholic Church: Gives an explanation of various practices of Catholics to make those practices more meaningful.
The Imitation of Christ: After the Bible, this is known as the second most read book in history. It gives us personal insight into one trying to live a life for the Glory and Love of God vs. being saved out of self interest.
One of the following: Dark Night of the Soul, Interior Castle, The Little Way. They each both draw on each other but give a different glimpse into discerning truer Holiness.
Something by a Doctor of the Church. Personally, I think Confessions by Augustine is good as it covers so many areas: Conversion, Praise, and theology. And, who can’t identify w/ the words of this prayer: “Too late have I loved you, O Beauty so ancient, O Beauty so new. Too late have I loved you!”
Something by Henri J. Nouwen, GK Chesterton, Bishop Sheen or Thomas Merton.
As much as possible on a person’s partron saint. If you have a perpetual prayer partner, you probably should know something about him/her.
Something by Pope JPII or BXVI. Because they write in contemporary language and about contemporary issues, they are easier to grasp relevance. Pope Benedict’s “Introduction to Christianity” is amazing. He took over 100 pages to just explain what it means to say “I believe” in the Creed.