The writers of the books of the Bible were just ordinary humans, but it is the historic Christian belief, that they wrote under the influence of the Holy Spirit; that is under 'inspiration'. This inspiration factor makes the Bible qualitatively different from any other book that exists - past, present or future. It therefore bears unlimited importance for us to study and understand. Many other books existed at the time the Bible books were chosen. The books that are in the Bible today, were chosen by persons spiritually guided by the Holy Spirit. This enabled them to choose what was spiritually true and what was false and therefore should or should not be included as 'scripture'. The collection of books recognized as authoritative and divinely inspired, were canonized. Canon is a Greek word which means rule or measuring line. The books that passed the canon were considered scripture. Other books that did not pass the canon, may be of interest to read, and suitable for learning, but are not regarded as on the same level as Biblical or 'canonical' books in that they carry no authority.
In Old Testament times, a canon of books was not needed, since a living prophet was usually present to announce the will of God for the people. When a prophet was not present, previous prophet's writings and teachings, as given by God (such as the Mosaic law) were collected for study and memorization. After the coming of Jesus, the old methods defined by the covenants of the Old Testament, were no longer in effect. Each person can have a personal relationship with Jesus, and being led individually by the Holy Spirit, a national level prophet is no longer available. We study the scriptures, and with the inspiration of the Holy Spirit, we can determine God's personal will for our lives.
At the time of canonization, numerous other books were purported to be 'inspired'. The canon was necessary to preclude the possibility of additions to the number of truly inspired works. This rule still holds today. The Bible is the single source of the written word of God.