This is a modern Christian classic, read by millions since its first publication in 1978. Whether you agree or disagree with the book in general, there is no doubt that it has influenced generations of Christians. Some of my friends love this book and recommend it freely, others won’t even pick it up themselves because they believe it is heresy, or very close to it.
Foster is a Quaker, and writes from that perspective. He is also steeped in Christian Mysticism and frequently refers to the authors of these books throughout. All of these disciplines have been drawn out of their earlier works, and it would be fair to say that Celebration of Discipline is a modern interpretation and summary of the best of the early Christian mystics work. The result is a very readable book, just a couple hundred pages, full of information on the disciplines of the spiritual life.
I found the book helpful in many ways, but also question the methods Foster uses to arrive at many of his conclusions. He frequently uses proof texting and takes verses out of context to back up his points, and some of what he writes is clearly wrong. Some of the things he suggests are just silly, and the organization of the book could use some improvement.
That said, I would recommend this book to most believers, provided they read it with a discerning mind. I found the most value in Foster’s practical tips on each discipline, and many of them have been immensely helpful. I learned a lot from reading this book and am sure many Christians would benefit. If nothing else, you should read it because it is so popular and has influenced so many. You might be surprised at what you find.
One other note: I’ve found that reading writers that I often disagree with is more rewarding than reading writers that I’m already on the same page with. Reading someone that you disagree with a quarter of the time keeps you on your toes and will help you engage with the topic further.