Bounds classics on prayer have been recommended for over 100 years.
"Nothing distinguishes the children of God so clearly and strongly as prayer. It is the one infallible mark and test of being a Christian. Christian people are prayerful, the worldly-minded, prayerless. It is the outward and upward flow of the inward life toward its original fountain. It is an assertion of the soul's paternity, a claiming of the sonship, which links man to the Eternal. It has everything to do with enriching, broadening and maturing the soul's experience of God. That man cannot possibly be called a Christian, who does not pray. By no possible pretext can he claim any right to the term, nor its implied significance. If he does not pray, he is a sinner, pure and simple, for prayer is the only way in which the soul of man can enter into fellowship and communion with the Source of all Christ like spirit and energy."
"Nothing distinguishes the children of God so clearly and strongly as prayer. It is the one infallible mark and test of being a Christian. Christian people are prayerful, the worldly-minded, prayerless. It is the outward and upward flow of the inward life toward its original fountain. It is an assertion of the soul's paternity, a claiming of the sonship, which links man to the Eternal. It has everything to do with enriching, broadening and maturing the soul's experience of God. That man cannot possibly be called a Christian, who does not pray. By no possible pretext can he claim any right to the term, nor its implied significance. If he does not pray, he is a sinner, pure and simple, for prayer is the only way in which the soul of man can enter into fellowship and communion with the Source of all Christ like spirit and energy."