The Spirit of the Disciplines by Dallas Willard
Dallas Willard is one of the towering geniuses in modern day theology, and his recovery of the importance of discipleship in the Christian faith is of monumental importance to the modern church. The Spirit of the Disciplines outlines Willard's argument for the importance of disciplines in spiritual formation and is an absolute must read for those serious about becoming Christ-like.
Willard's book is a clear, well-reasoned discussion of the Christian hope, the importance of the disciplines to that hope, followed with analysis of the nature and purpose of the major historical disciplines. This is not, however, an outline of the actual practice of the disciplines, for that Willard recommends the reading of Richard Foster's Celebration of the Disciplines.
I actually read most of the book some time ago, but because of starting school I had not finished the last few chapters and it's actually the last two chapters constitute some of the few flaws in this work, though for their placement rather than their content. The last chapter considers the power structures of this world, and I think it should have been part of the introduction to the book, while the second to last chapter concerns poverty and feels like it should be an appendix to the book, as it doesn't really flow with the main discussion.
Finally, while I do agree with Willard about the immense importance of the disciplines, I think he gives a bit too much credit to what they can accomplish. That is to say, while I think the disciplines are of immense importance in creating follows of Christ who live their lives in the presence of the Kingdom, he seems to imply that implementing them would by itself actually bring about the full and final ushering in of God's Kingdom and the return of Christ. I actually doubt that this is what Willard himself believes, but it is what comes across in the final chapter of the book.
So, while I've been saying this about a lot of books lately, I recommend that every Christian serious about their faith should read and understand this book.