I just concluded a little book on prayer, David McIntyre (1859-1938), The Hidden Life of Prayer: The Life-blood of the Christian (Glasgow: Christian Focus, 2010). It is brief, only 123 pages long. It was first published in 1891. It will challenge you in the area of your prayer life. It is filled with excellent quotes and helpful anecdotes. McIntyre was clearly well read. He approached prayer as not only a duty but an invitation and a necessity for spiritual vibrancy. He gives this very useful meaning of prayer,”Prayer is the avowal of our creature-dependence” (p. 106). It will be well worth your time to read this excellent little work.