This book helped to clarify some of my thoughts on the effectiveness of prayer.
I especially benefitted from Hiebert's conception of prayer as a three-step process: 1. God invites us to pray. This invitation is found throughout Scripture. 2. We respond to this invitation by coming to his throne with our prayers. 3. God responds to our prayers with his answers.
Thus, prayer is working with God as it is a two-sided operation.