I think that most of these are available free online and thus meet the definition.
25 Books Every Christian Should Read:
1. On the Incarnation by St. Athanasius
2. Confessions by St. Augustine
3. The Sayings of the Desert Fathers
4. The Rule of St. Benedict by St. Benedict
5. The Divine Comedy by Dante Alighieri
6. The Cloud of Unknowing by Anonymous
7. Revelations of Divine Love (Showings) by Julian of Norwich
8. The Imitation of Christ by Thomas à Kempis
9. The Philokalia
10. Institutes of the Christian Religion by John Calvin
11. The Interior Castle by St. Teresa of Avila
12. Dark Night of the Soul by St. John of the Cross
13. Pensées by Blaise Pascal
14. The Pilgrim’s Progress by John Bunyan
15. The Practice of the Presence of God by Brother Lawrence
16. A Serious Call to a Devout and Holy Life by William Law
17. The Way of a Pilgrim by Unknown Author
18. The Brothers Karamazov by Fyodor Dostoevsky
19. Orthodoxy by G. K. Chesterton
20. The Poetry of Gerard Manley Hopkins
21. The Cost of Discipleship by Dietrich Bonhoeffer
22. A Testament of Devotion by Thomas R. Kelly
23. The Seven Storey Mountain by Thomas Merton
24. Mere Christianity by C. S. Lewis
25. The Return of the Prodigal Son
thanks.. posted your list on FB & gave you the HT. I added a few more of my favs