For young kids
The Garden, the Curtain, and the Cross by Carl Laferton
For elementary school kids
St. Patrick's Summer by Marigold Hunt
For middle school kids to adults
The Imitation of Christ by Thomas A' Kempis
For high schoolers to adults
Theology for Beginners by Frank Sheed
For adults
The End of the Present World and the Mysteries of the Future Life by Fr. Charles Arminjon (also recommended by St. Therese of Lisieux!)
Forming Intentional Disciples by Sherry Weddell
The Privilege of Being a Woman by Alice von Hildebrand