The Didache, or "The Teaching of the Twelve Apostles," is probably the most important church document outside of the Bible itself. Why? Largely because of its extreme antiquity. While we have some of the writings of the Church Fathers (for example Clement of Rome) that date to the end of the first century A.D., the Didache is now dated to that same period. This makes it, along with Clement's letters, the oldest Christian documents outside the New Testament.
What makes the Didache of such great importance as well is its authoritative nature. Rather than being a letter written by an individual bishop, the Didache is an authoritative manual for the Church community. It was also highly esteemed by the early church, even being considered by a few to be a part of the New Testament canon.
The value of the Didache also lies in the invaluable insights it gives into the beliefs and practices of the early church. This, combined with its very early age, make it worth the time for any serious Christian to study, if they want to understand not only the early Church but also the New Testament better.
The first section deals with "The Two Ways" (the way of life and the way of death) and occupies the first 6 chapter. But it's some of the rest of the material that is especially valuable. Chapters 7-10, for example, give us a rare and valuable insight into the practices of Baptism, fasting, praying, and the Eucharist of the early Church. Having been written so close to the time of the apostles and the writings of the New Testament, these sections shed exceptional light on what the early Church believed and how it practiced the faith. Though the Didache's instructions for the Lord's Day are brief, they clearly demonstrate that the Eucharist was the main service of the Church on the Lord's Day.
The Didache is surprisingly short and is well worth the time you spend reading it. While there are a lot of versions out there, this Kindle version is a convenient one. However, there is only a single paragraph of introduction, and so I suggest supplementing it either with some reading about the background and history of the Didache, or possibly buying a different edition with a more complete Introduction.