Any book that can teach Christians to pray the Lord's Prayer more frequently, more thoughtfully, and more fully is, in a sense, a great book.
Packer's teaching on the Lord's Prayer in "Praying the Lord's Prayer" is excellent. He divides his teaching on the Lord's Prayer into 16 short chapters, so it's a very user-friendly book. He spends time on each phrase, as well as providing a few introductory chapters, and even a chapter on "Amen."
Packer writes in a very simple yet deep way that will be a benefit to just about any Christian, young or old, young in the faith or a pastor, teacher, or parent. One of the things Packer does best is to break down the structure of the Lord's Prayer and show how the parts relate. And then he explains each phrase in a way that gives you something to think about. Packer is also good at providing some mnemonics for remembering key points. For example, "provision, pardon, and protection" - 3 ideas that are central to the Lord's Prayer.
This is the secret to praying the Lord's Prayer: to meditate on it in slow motion and phrase by phrase so that whenever you pray it is filled with rich meaning in Christ for you. Unfortunately, I think that most Christians who pray the Lord's Prayer simply pray it when in church and try their best to think about something for the 23 seconds it takes to pray it. This isn't a very fulfilling way to pray! What if, instead, you had already deeply meditated on the meaning of each phrase, as well as how they work together?
"Praying the Lord's Prayer" is a great little book.
The only reason I'm giving the book 4 stars is because "Praying the Lord's Prayer" was extracted from Packer's larger work, "Growing in Christ." For only $1.50 more or so you can get not only Packer's teaching on the Lord's Prayer but also his teaching on the Apostles Creed and the Ten Commandments. I've used "Growing in Christ" in a confirmation class I teach because the Lord's Prayer, the Apostles Creed, and the Ten Commandments are the foundations of the Christian life.