Rhythms of the Inner Life is a great book by f/Friend Howard Macy. It's been available on paper for a while, but it just came out as an e-book! Check it out, it's only $3.99, and well worth it.
Howard explores the Psalms as a way of helping us understand the rhythms of our own inner lives, focusing on seven ways we connect with God, as witnessed in the Psalms: longing, waiting, trembling, despairing, resting, conversing and celebrating.
Although this book wasn't written just for Quakers, it speaks with Quaker sensibility, inviting us to meditate on the ways we interact with God, and encouraging us to not just think about it but to actually do it. There are also some group (or individual) study questions, so you could use it with a small group or a worship group. The study guide has suggested readings from the Psalms and the New Testament, suggested hymns based on the Psalms and questions for discussion or personal pondering.