12 is a key number in the Bible - e.g. 12 tribes of Israel, 12 disciples of Jesus. In the current organization of the Bible, the 12 most influential books seem to be:
4 Gospels (Matthew, Mark, Luke, John)
I guess the only book above that needs explanation is "Deuteronomy" - I included it after reading somewhere online that out of the current Old Testament Books, the top 5 most "quoted" in the New Testaments are what I have included; in fact, Deuteronomy is the 3rd most quoted, ahead of Genesis or Exodus. And Jesus' answer to the devil in his temptations is presumably from Deuteronomy.
Why all 4 Gospels? One reason is that the Christian Church really couldn't decide which one should be excluded! Now Matthew is supposed to be most quoted by early Church Fathers; John is the only one outside the "synoptic" gospel (and one article says arguably the whole Christian trinitarian theology arises out of John's Gospel, which I find insightful); Luke could have been originally the same book at the Acts; while Mark was the first Gospel to be established. So no easy way to just say read 1 or 2 or 3.
A suggested reading order?
2-4. Luke, Acts, Romans
5-7. Genesis, Exodus, Deuteronomy
9-10. Isaiah, Matthew
11-12. John, Revelation