If you take a moment to think about all God has told each believer to do, it is almost "Go into all the world and preach the gospel; feed the poor; cloth the naked; lay hands on the sick; visit the prisoner" and so much more. Fulfilling this commission can only come from our relationship with Christ and that relationship is developed primarily through prayer.
In many ways this book taught me how to pray! That is a pretty bold statement since I have been praying for as long as I can remember. I have re-done this book many times, sometimes twice a year since the initial read. I say re-done because you don't just read this book, you do it, and when you are done you feel remarkable. The concept is over the course of 40 days straight, you pray for forty-minutes everyday, but not for yourself, you intercede for others. It's a powerful process, but I would be lying to say it can be difficult sometimes.
This is a good starting point for new believers who are learning to pray and spend time with God. Also serve as a good reminder/refresher for any believer who wants to strengthen their prayer life. Highly recommend this book for all believers.