The "Experiencing God" book is actually a 13-week study. It's all about how to know and do God's will. It's Bible based, so there's a lot of opportunity for God to speak to someone to call them or affirm their calling. It's full of Biblical and historical examples of how men and women responded to the Lord's call upon their life and how He did amazing things through them. Besides the Bible, "Experiencing God" has been the way through which God has spoken to me the most.
Blackaby, Henry and Richard. Experiencing God: Day by Day Devotional.