Dec 27- Doctrine and Covenants 1

It is pretty astonishing to think about the fact that the rest of the Christian world uses the Bible, which we love and revere, but that as members of the church we have 3 extra books of scripture. It is made clear in the title page of the Book of Mormon the purposes of it being brought forth. But what about the Doctrine and Covenants? What is the purpose? Why was it given and recorded and what was the process of it coming to be?