Prayer
1 - Beginner
A Way to Pray: A Biblical Method for Enriching Your Prayer Life
Matthew Henry (edited by O. Palmer Robertson)
One of the most important things any Christian should learn is to pray the Scriptures. What does that mean? It means to read and mull over the meaning of Scripture passages, and then to move those passages from the page into the heart and to the tongue where we pray what God has said. There is simply no one who models this for Christians better than Matthew Henry. I highly recommend readers get the Banner of Truth edition that was edited by O. Palmer Robertson because of its readability.
2 - Intermediate
Cloud of Witnesses: A Treasury of Prayers & Petitions Through the Ages
edited by Jonathan Arnold
This collection of prayers for different occasions from church history is another great help to the Christian who wants to see prayer modeled, and to have help in their own prayers. I am a big supporter of using written prayers as a springboard to our own prayers.
2 - Intermediate
Prayer: Experiencing Awe and Intimacy with God
Tim Keller
Even for those who are not generally interested in reading Tim Keller, they would be greatly helped to read this wonderful book on prayer. It is probably my favorite all around general-purpose book on prayer. It not only encourages the readers to cry out to God, but also draws solidly from church fathers and puritans for its substance. This is a mature and balanced work that will nudge the prayerless back toward a God-dependent posture.