Well, as a Christian I believe that the Bible is a book that contains knowledge of the one, true, living God-- the God of Abraham, Isaac, and Jacob. I believe that the book I know as the Old Testament is a revelation of God: his creation of the world, the ways he wanted us to live by, his reaction to us when we choose not to follow those ways, etc.
I believe that the gospels of Matthew, Mark, Luke and John should be added to the Bible because these are the books that speak of Jesus' life, and since I believe that Jesus is the messiah, his life is an important revelation of God.
Though other Christians believe whole-heartedly in Acts and in the letters as being the fully-inspired word of God, I believe that they are letters written by early Christians and based on their experiences as Jesus' first followers. I believe that they are written to advise and to encourage Christians in their lives, and that the authors had an extremely close relationship to Christ (i.e. had been his disciples, or had received visions and empowerment by the Holy Spirit), but personally I am critical about these letters, and don't put an equal amount of importance on them as I do the rest of the Bible; the choice of these letters was by consensus, and some were left out. I don't place as much importance on the book of Acts, either, though it does make for an interesting comparison between my church and the first church.
I believe that the Revelation of Jesus Christ is the final prophecy given to us by God before the end of the age, and basically that means there is nothing else that really needs saying until the prophecy is fulfilled, and Christ returns to establish his kingdom. Just like there was a gap of about 400 years between the last of the OT prophecies and the birth of Christ, there is now simply a gap between Revelation and its fulfillment. Adding anything would be superfluous.