Eric Metaxas has written a terrific biography of Dietrich Bonhoeffer. Reading it (which I highly recommend) has prompted me to pull my books by Bonhoeffer off the shelf (alas, I only have two) and read the man himself. His little prayer book on the Psalms is rich and rewarding and prods me to do more than just read about prayer:
The phrase "learning to pray" sounds strange to us. If the heart does not overflow and begin to pray by itself, we say, it will never "learn" to pray. But it is a dangerous error, surely very widespread among Christians, to think that the heart can pray by itself. For then we confuse wishes, hopes, sighs, laments, rejoicings--all of which the heart can do by itself--with prayer. And we confuse earth and heaven, man and God. Prayer does not mean simply to pour out one's heart. It means rather to find the way to God and speak with him, whether the heart is full or empty. No man can do that by himself. For that he needs Jesus Christ.