score:13
The "books" of the Bible are just that- they are separate works by separate authors in separate contexts. Yes, Luke wrote 2, Moses wrote 5, and Paul wrote somewhere between 7 and 13, but Stephen King has written more than one book, too :)
Matthew, for example, was written by the Apostle Matthew. Luke, by Paul's traveling companion. Mark was written by Peter's close friend. The point is, these are different books by different authors that are just assembled into a library of what, by consensus, has been agreed to be the best of the best.
To ask, "what books dind't make it in?" is answered by a simple statement: everything else.
There were all sorts of Gnostic Gospels at the time, but, of course, the main stream of Christianity rejected that. As such, there was never a "conspiracy" to keep out, say, the 115 proverbs that are the Gospel of Thomas, or the Epistle of Barnabas, or the Gospel of Peter, or Judas, or any of the lot. There was simply a general agreement that all of these books had no more place in the Christian canon than Mitt Romney would in the Democratic Convention.
Additionally, there were several fictional works, in the same way that today people write historical fiction. Google "New Testament Apocrypha" for details.
Finally, one accepted criteria for canonicity was an apostolic claim. As such, important Christian works such as 1 Clement, the martyrdom of Polycarp, The Shepherd of Hermas, or the Didache are highly important and good theology in them. They are worthy of study by Christians, but never made the criteria of canonicity.
In short, the Bible is really a lot like, say, the New York Times best seller list. It is a compilation of what's "best". What's not included isn't trash- its just not in the same league. The same is true of the canon. C.S. Lewis is amazing, but he isn't the agreed upon historical consensus.