This book is about how to pray as Jesus taught us: "Your Kingdom Come."
With the death, resurrection and ascension of Jesus, the human (and divine!) king has been enthroned and is at work destroying all his enemies. The kingdom has come, but not fully. That will happen at the second coming of Christ. In the meantime, the kingdom is coming.
But the prayers of God's people are a significant factor in that coming. Prayer is a vital link between the present stage of the kingdom its final stage. God is sovereign, of course, and has an eternal plan. At the same time, however, he uses the prayers of his people to advance his kingdom. If you don't pray, he may use the prayers of someone else, and you may lose the opportunity and privilege to participate in its coming.