Okay, start here:
As Christians, how is it that we know we are saved by the death and resurrection of the incarnate Son of God? For those raised as Christians, the Sunday School sing-song answer “for the Bible tells me so” may come to mind, and this fairly well summarizes the Protestant teaching on the communication of saving truth. The Belgic Confession, an historical expression of the Reformed faith used widely in Dutch denominations, asserts that we know God by the beauty of creation, and “more openly by his holy and divine Word.” 1 The Westminster Confession of Faith, widely adopted by Presbyterian denominations with traditionally Scottish origins, contains a comparable teaching: while the “light of nature, and the works of creation and providence do so far manifest the goodness, wisdom, and power of God, as to leave men unexcusable,” we still need revealed truth to possess the “knowledge of God, and of His will, which is necessary unto salvation.” 2 Regarding this revelation, the Westminster Confession holds that God chose “to commit the same wholly unto writing.” 3
And then, go here:
The question How do you know what books belong in the Bible? is a significant one indeed. The Catholics and Orthodox will normally refer to the establishment of these books as part of the canon by fourth century councils. Catholics would further refer to the teachings of the council of Trent (1545-1563) which dogmatically and infallibly declared the current Catholic canon (including the Apocrypha) as being authoritative.
Do we have a solidly fixed canon?
I’ve been skirting around the issue for a while, and I’ll keep flirting with danger, but in my opinion – you want books which serve to connect the Old with the New? Wisdom, Sirach and 1st Maccabees. Oh, and the Psalms of Solomon.