This is a book which Professor Cullmann, now in his nineties, has long been wanting to write; its profundity and scholarship show that has not lost any of the qualities which made his other New Testament studies so memorable. But he has always been more than just a biblical specialist in his desire to relate his findings to the experience of being a Christian today, and in this book he foes even further in that direction than before.
Cullman's work is marked by his depth of research and careful attention to the text. It is also quite concise.
He articulates some aspects of NT prayer helpfully, such as the role all three persons of the Trinity play.
As a negative, this book suffers from the blight of historical criticism, which, even when Cullman rejects the false conclusions put forth by those methods, stains his overall approach. The basic assumptions behind that approach are wrong.
Cullmann does a magnificent job of drawing out the meaning of the pertinent verses in the New Testament on prayer. What is most valuable about this book, however, is the way he allows his exegesis to become a conversation partner with questions posed by the contemporary person about prayer (i.e. The Immutability of the Divine Plan and the Divine Freedom to Hear Prayers, Prayer and the Question of God's Omnipotence, etc.). This book is insightful throughout and provides the most reliable guide on the topic of prayer in the New Testament that I know.