I have been reading this book since summer (obviously taking my time). God had shown me that He wanted to use me to intercede for others in prayer, & He taught me a lot through this book.
Yancey discusses several different aspects of prayer, such as "Why Pray," "Does Prayer Change God," "Unanswered Prayer," & "Prayer & Physical Healing." He has lots of stories & perspectives of other people regarding prayer in their lives, & did a lot of research about the history of prayer in the church. I like that he always brought it back to the fact that prayer is really just conversing with God, the Creator, the Giver & Sustainer of life.
I have never felt like my prayer life was what God wanted it to be, & I was encouraged by this book & I think it gave me a better picture of prayer in my life. Of course since I took nearly a year to read it, I can't remember a lot of specifics about it, but I did like it, & if the subject of prayer interests you, I would suggest reading it.