Was the bible important to God's plan? Yes, it is important. Scripture has always been important and there is plenty of evidence from throughout history as to the importance of scripture.
Why didn't he make sure that everyone got to read one?
Reasonable levels of literacy are a figment of relatively recent history, so personal reading of the bible would not be something that would have been relevent historically. What we do know is that there was much reading of the bible by those who could read to those who couldn't. If you read the Gospels, including Acts, and the Letters, you will see a lot of references to Old Testament scripture in the teaching. The people did get to hear scripture, even if they couldn't read it themselves. Within the Christian church, bible reading is very important. I don't know of any denominations that do not have at least one bible reading in their services. Some churches have three or four readings in each service, as well has many quotations from the bible in spoken liturgy. Hymns and songs of praise also rely heavily on biblical quotes.
We only know for sure about God's will for his people (all people) because of what it says in the bible. We can infer beyond that because the nature of God is revealed in the bible. We know what plans God has for those of us who have made a decision to leave our old lives behind and follow Jesus. We do not know for sure what his plans are for those who have never, ever heard of him.
Because we know what God wants us to do, and he has commanded us to tell others about him, then that is clear that this is what we need to do. God uses his followers to reach out to other people. He may also use signs and wonders but we don't know for sure because the bible does not tell us everything that we want to know, just everything we need to know for salvation. We would not expect God to leave copies of the bible under some kind of gooseberry bush for unwitting people to discover - God uses us to do his earthly work. Besides, everyone who lives in a relatively free country does have access to a bible by visiting a bookshop!
Revelation through the bible is one way that God develops a relationship with people. It is not the only way. And while it may be the most important way for many Christians today to learn about the nature of God, it is not necessarily the same for everyone.