Books about Why We Pray

In the trending discussion (if you can call me posting and talking about a trend) on books that might be useful in teaching and inspiring tefilla I have come across the following English selections:

• Why We Pray What We Pray by Rabbi Dr. Barry Fruendel (2010)
• Praying with Fire 1 by Rabbi Heshy Kleinman (2006)
• Call to the Infinite by Rabby Aryeh Kaplan (1986)
• To Pray as a Jew by Hayim H. Donin (1991)
• A Guide to Jewish Prayer by Rabbi Adin Steinsaltz (2002)

What else should or would you add to this list?