Interestingly enough, the book of Romans informs us that all we need to know about God has been revealed to us individually as part of our created essence, or nature, or being.
For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men who suppress the truth in unrighteousness, because that which is known about God is evident within them; for God made it evident to them. For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse.
According to Christianity, we all innately know there is an eternal God, and we all know that we ought to behave in certain ways as a result.
Yet we choose not to.
Further, since we all innately know that God is the one in charge, we all know that really only He can fix the problem. This innate knowledge alone should put us on our knees begging for His assistance, His forgiveness, and His mercy.
No book necessary.
The book in question chronicles how God has interacted with Man over the ages. It tells us specifics about creation and God's condescension to be intimately involved with man. It further tells us the good news of specifically how God has graciously decided to redeem us.
This is why the Gospels are called Gospels -- the word means "good news".
In other words, the vital and critical message for man exists innately within us. That message is, essentially, that we are sinners in the hands of an angry God. Since we all have that message innately yet continue in sin, we are all without excuse.
The Bible has been offered as "good news" to God's elect. It brings with it a breath of fresh air for those of us who trust in Christ, a message that we can be forgiven. The Bible also reinforces the message that we are condemned without God's forgiveness, making it good news to the elect but further condemnation to those who are perishing (1 Corinthians 1:18; 2 Corinthians 2:15-16).
In sum, the Bible offers additional information. All we need to know, we already innately know.
So why did God share this good news with a book?
Why not? Books are one method that God has caused people to use to understand ideas for ages. When God communicates with people, He communicates to us in ways that He has caused us to be able to understand communication. Books are one of those ways. Sometimes we hear Him speak to us audibly in our own language. Sometimes he sends personal emissaries to speak to us (much as we often do ourselves) such as angels, prophets, or apostles.
When communicating with people, God communicates in the ways we understand.
If you have recognized that you are not perfect (Romans 3:23), all that remains is for you to ask the God you know exists to forgive you and save you. If you do this sincerely, He will hold fast to His promises in His book of good news:
If you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved; for with the heart a person believes, resulting in righteousness, and with the mouth he confesses, resulting in salvation.