The book Celebration of Discipline by Richard J. Foster is about the process of discipleship and how to grow in one's relationship with God. It discusses the importance of various spiritual disciplines such as prayer, fasting, meditation, and study in order to mature spiritually. The author provides insights and guidance on how to cultivate these disciplines in one's life so that they can become second nature. Ultimately, the goal is to develop a deeper intimacy with God which will lead to greater joy, peace, and fulfillment.