The Bible is so much more than a historical record of evidences of God dealing with mankind. It is primarily a spiritual book written by God himself which contains the message of the gospel all throughout its historical records.
Romans 7:14 (KJV)
14 For we know that the law is spiritual: but I am carnal, sold under sin.
The law is the whole word of God and it is spiritual. This means that anyone can pick up the Bible (saved or unsaved) and read it and learn things from it, like they can learn historical facts as well as moral truths. But only the true children of God have been given eyes to see and eras to hear the spiritual truths which God has concealed within those historical accounts and moral truths. Christ puts it this way:
John 8:43-47 (KJV)
43 Why do ye not understand my speech? even because ye cannot hear my word. 44 Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it. 45 And because I tell you the truth, ye believe me not. 46 Which of you convinceth me of sin? And if I say the truth, why do ye not believe me? 47 He that is of God heareth God's words: ye therefore hear them not, because ye are not of God.
Matthew 11:15 (KJV)
15 He that hath ears to hear, let him hear.
This is the reason Christ spoke in parables all the time, to conceal truth from the unsaved.
Matthew 13:10-13 (KJV)
10 And the disciples came, and said unto him, Why speakest thou unto them in parables? 11 He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given. 12 For whosoever hath, to him shall be given, and he shall have more abundance: but whosoever hath not, from him shall be taken away even that he hath. 13 Therefore speak I to them in parables: because they seeing see not; and hearing they hear not, neither do they understand.
So, as far as its usefulness, it depends on whether the person is truly saved or thinks he is, but in fact isn’t. Each will focus on different things when they pick up the Bible. The purpose of having the Bible is so that we can have all of this (historical, moral and spiritual) information revealed to us by Christ the creator. And as far as trustworthiness, the Bible is the only book that can be trusted in matters of this creation, salvation and morality. God has taken it upon himself to preserve his word perfect and pure, free from corruption.
Psalms 12:6-7 (KJV)
6 The words of the LORD are pure words:
as silver tried in a furnace of earth, purified seven times.
7 Thou shalt keep them, O LORD,
thou shalt preserve them from this generation for ever.
I hope this helps.