The Old Testament was mostly about the covenant God made with Abraham so that he could return to operating in the Earth after Adam caused the fall of man. God gave dominion over the planet to Adam and when Adam sinned Satan became the god of this world. God made the pact with Abraham so that he could establish his own people that would eventually bring Christ into the world.

The New Testament is the culmination of those plans across millenia. The rules in the Old Testament were more strict because salvation wasn't possible and anyone that strayed too far was unredeemable. Since the blood of Christ made true redemption possible and unseated Satan as the god of this world, the rules changed.

As far as accuracy, the Bible was written by men under the inspiration of the Holy Spirit. You won't find errors. You have to remember, though, that the Bible was written in several languages and then translated into English. And that translation was done quite a long time ago. So sometimes you can read the Bible in English and miss the entire meaning that would be clear if you read it in the original language.