In order of when I stumbled across them.
1. The Bible (specifically the New Testament) St. Anthony's Guild version. Acquired from "Friends of the Library" discard sale, Belmont, MA, 1974. 10 cents.
The New Testament is truly new. It is the antithesis of what we would normally think/do, and yet it is the true fulfillment of our desires.
2. The Imitation of Christ by Thomas a Kempis (found in basement of my house)
devotional, instructional, challenging and comforting
3. Man and Woman He Created Them--A Theology of the Body by John Paul II the Great (proofread the first English edition when I entered Daughters of St. Paul)
The most revolutionary book after the New Testament, because it arranges all of salvation history according to the body. It arranges the Bible according to the body. It arranges philosophy according to the body. It arranges life according to the body.
4. The Practice of Humility by Pope Leo XIII
5. Fides et Ratio (Faith and Reason) by John Paul II the Great
6. The Humility and Suffering of God by Francois Varillon (translated from French)
References only things French. You must like French things to read this book.
7. Power in Praise by Merlin Carruthers
I read this after joining the charismatic renewal. It teaches you very simply to praise God for everything. It teaches you to praise God for the bad things in your life in particular, because you believe He is working in them RIGHT NOW to bring them to a good end for you. All Carruthers' books have the word "Praise" in the title, and I can vouch for "Prison to Praise" (his life story), and another marvelous book called "Bringing Heaven into Hell," the reading of which partially cured a friend of mine from mental illness. Enough to get ot of an institution.
8. I Believe in Love by Fr. D'Elbee
Unpacks St. Therese's "little way." Will scour the last vestiges of Jansenism and Manichaeism out of you. If you ever have wavered or ever do still waver in your belief in God's love for you, this book is for you.
9. The Caged Virgin--An Emancipation Proclamation for Women in Islam by Ayaan Ali Hirsi
Born into an Islamic family, Hirsi fled the Sudan and became a member of the Dutch Parliament. While doing a movie about women in Islam, her collaborator, Theo Van Gogh (a relative of Vincent Van Gogh) was assassinated in broad daylight (stabbed with a machete). The machete pinned a note to his body explaining that he was killed for working on this film. Hirsi was given asylum in the USA where she wrote her life story: "Infidel." She racked up a $2M bill for 24-hr security, and the US informed her that we could no longer provide security, so her fellow atheist/author, Sam Harris is working with Rick Warren ("The Purpose-Driven Life") to find a way to continue to protect her life.
10. The Wet Engine by Brian Doyle (A book that haunted and pursued me--just by its title, until I bought it and read it. Now its contents haunt and pursue me.)
One of the most beautifully written books ever, about our hearts, our bodiliness. Poetically scientific, scientifically poetic. Funny. Small, hardcover, gorgeous cover design. Great for anyone who ever had heart trouble. Very Irish. Very Catholic. Very Irish Catholic. Lends itself to repeated readings aloud.
Anything by Dave Barry can be pretty life-changing, too. Also, the poems of Hafiz and Kabir (Islamic poets/mystics who are more direct, intimate and funnier than Christian mystics.)