Prayer: Finding the Heart's True Home by Richard Foster. In this Ephesians 6:18, the apostle Paul writes, "And pray in the Spirit on all occasions with all kinds of prayers and requests." In this classic work on prayer, Foster explores twenty-one different types of prayer giving us some idea what Paul might have meant by "all kinds of prayers." Drawing on a variety of Christian traditions, Foster's list will likely include some prayers you are familiar with and some that are new to you.
Prayer: Does It Make Any Difference? by Philip Yancey. I started reading Philip Yancey's work in college. His writing is purposefully accessible to anyone regardless of where they are on their spiritual journey. At the same time, he blends this accessibility with a willingness to press readers just beyond their comfort zone. He continues this pattern in his work on prayer exploring many of the questions we have about prayer but are too afraid to ask.
Check out a fuller review I wrote for The Baptist Standard.