There are already several good reviews of this book, so I'll be brief.
I've had this book on my Kindle for a while, but I've never made the time to read it until yesterday. Let me say that, although I didn't know it at the time, this was exactly the book I needed to read at that time. The Lord spoke to my heart in multiple ways about prayer and its work. To prove my point, the book is short (it took me about 45 minutes to read carefully) but I highlighted 21 separate sentences or paragraphs for future use. There are many long books that don't get half that many highlights!
This is a very, very good book. I plan to buy physical copies and use it to facilitate a Bible study on revival praying sometime in the future. You will be better for reading this book.