Very happy to note that this month’s (May 2015) free Logos book is N.T. Wright’s handy little devotional book called The Lord and His Prayer. I am teaching a course on prayer next year and I have found this book helpful in my preparation.
Also, for $1, you can add his Following Jesus: Biblical Reflection on Christian Discipleship. I have used Following Jesus as a course textbook for undergrads in the past and many found it insightful and edifying. Enjoy!