R.A. Torrey 
Keep praying until God answers [EPUB ebook] 

There are two passages in the Gospel of Luke which throw a flood of light upon the question, ‘What sort of praying is it that prevails with God and obtains what it seeks from Him?’ and also upon the question, ‘Why is it that many prayers of God’s own children come short of obtaining that which we seek of God?’
This message will invite you to pray without ceasing.