A Long Obedience in the Same Direction by Eugene Peterson
This classic is a study of the Psalms of Assent (120-134) encompasses prayer, Bible study and discipleship.
Great for anyone who desires a deeper walk.
There is a separate 6-week study guide called Perseverance: A Long Obedience in the Same Direction that can be used by small groups. ($7.99 at Christianbooks.com)