I just finished Richard Foster's best known book 1978's Celebration of Discipline, which examines the inward disciplines of prayer, fasting, meditation, and study in the Christian life, the outward disciplines of simplicity, solitude, submission, and service, and the corporate disciplines of confession, worship, guidance, and celebration.
This book was great in that it appealed to my more traditionalist bent, but did so without much in the way of dogma. Foster takes the best from many different christian spiritual disciplines and puts them together to present what I would call a rule of life. In many ways it is a challenging read, in that it expects a lot from you. However, I tend to appreciate a religious book that doesn't pull punches. I got so much out of it that I have moved on to another of his books, Prayer: Finding the Heart's True Home.