Throughout the book Lewis makes these small, effortless statements that made me put the book down, take off my glasses, rub my forehead and stare out the window off into the mountains thinking, 'What in the world does that mean?' Or, 'There are so many implications connected to that simple statement.' And my head would begin racing (or just hurting).
Lewis comes at prayer from a base of God's immutable timelessness. So in order to agree with him on some of his thoughts on prayer, it may be important to agree with him on that aspect of God's character. (See also my entry on Terrance Tiessen's Providence & Prayer.)
What I appreciate most in Letters to Malcolm are Lewis's insights, or what he calls "festoonings" concerning the Lord's Prayer. This I found to be very practical for the ways in which Lewis applies Christ's instructions on prayer to his life: What does it mean to pray, "Thy Kingdom come," and, "Forgive us our debts as we forgive our debtors"?
I'm a fan of Lewis, and I enjoyed his mind-stretching commentary on prayer. It is encouraging that a mind like C.S. Lewis finds prayer to be just as mysterious as I do.