I've mentioned this book the most in my current series on prayer.
George Müller's life is a powerful answer to modern skepticism. His name has become a by-word for faith throughout the world. In the early 1830s he embarked upon an extraordinary adventure. Disturbed by the faithlessness of the Church in general, he longed to have something to point to as 'visible proof that our God and Father is the same faithful creator as he ever was'. He was more successful than anyone could have believed possible and is as much an example to our generation, as he was to his. (Amazon)
Another resource I've found helpful in my prayer series is this book by Andrew Murray.
Pastor Andrew Murray uses 31 chapters in With Christ in the School of Prayer to help readers learn how to change their lives and their relationships with Jesus Christ through the power of prayer. This guidebook teaches Christians not only how to connect with their Lord, but the true reasons it’s important to do so. Because of its assertion that prayer is a way of communing with God, rather than a chore or a trick to win favor, With Christ in the School of Prayer remains a valued classic for all Christian bookshelves. (Amazon)
E.M. Bounds has probably written more than anyone else on prayer. This book starts off addressing the importance of leaders to pray, then moves into the importance of everyone praying. I did not draw a lot of my sermon material from this book, but it was a very convicting personal reminder on the importance of prayer.
Power Through Prayer teaches readers that prayer is much more than just another thing to cross off their lists each day. This guidebook provides believers with information about the most effective ways to use prayer to better understand God’s word, fully appreciate divine power, and more deeply commune with God. Power Through Prayer has the potential to transform a static but faithful Christian life into one that is dynamic, ever-changing, and always learning. (Amazon)