I want to touch on the issue of canonicity, briefly as I can, before I wrap up my series (God willing, tomorrow.) This is an issue of some importance because we need to have a general understanding of why some books are included in the Bible and others are not.
This site talks about the importance of canonicity better than I could. Basically, it is important because what is included in the Biblical canon is considered, by Christians, authoritative. If it's not "in" then it doesn't carry the same weight that the Biblical texts carry. There is sound reason for this. For one thing, if any book describing Jesus were to be considered authoritative, we would run into the problem of comingling legitimate texts with heretical texts; blasphemy would be considered on equal footing with the truth, at least from the orthodox Christian perspective.
So how do we "know" that these texts we have are, in fact, the ones God wants us to rely on as His authoritative word? The article above touches on that from a spiritual perspective. We don't so much "know" as we "trust." We trust that the Holy Spirit guarded these works and guided those early church leaders who compiled the canon (for specifics on how the OT canon came to be, see here, and for the NT canon, see here.) These leaders also relied on the earliest church leaders to vouch for which NT books were considered authoritative, as these men were in direct lineage of the Apostles themselves. Again, a matter of trust that the church fathers knew whereof they spoke. (This is a reasonable assumption, I think, but it is an assumption nonetheless.)
This trust was built on the Holy Spirit, of course, but also developed through a rational examination of the arguments for/against a given text's inclusion. This doesn't mean that the decision was applied universally (cf the Apocrypha, for instance), but for a given church, the process for arriving at the canon chosen was not a matter of preference, but of rational analysis and trust in the faithful transmission of authority from the Apostles themselves.
During this series, I have touched on the reliability of the canon we have. The fact that it appears, rationally, to be reliable is a testament to how well the leaders of the early/middle-age church did their job insofar as the canon is concerned.