D. L. Moody 
Secret Power — or the Secret of Success in Christian Life and Work [EPUB ebook] 

One man may have “zeal without knowledge, ” while another may have knowledge without zeal. If I could have only the one, I believe I should choose the first; but, with an open Bible, no one need be without knowledge of God’s will and purpose; and the object of this book is to help others to know the source of true power, that both their zeal and their knowledge may be of increased service in the Master’s work. Paul says, “all Scripture is given by inspiration of God, and is profitable;” but I believe one portion, and that the subject of this book, has been too much overlooked, as though it were not practical, and the result is lack of power in testimony and work. If we would work, “not as one that beateth the air, ” but to some definite purpose, we must have this power from on high. Without this power, our work will be drudgery. With it, it becomes a joyful task, a refreshing service.
May God make this book a blessing to many. This is my prayer.
L. MOODY.