What is the most important book in the Old Testament?
This is a very hard question to answer because every part of the Old Testament, every book, has its place and helps us to understand God’s mission to restore his relationship with sinful humanity. I don’t know that I could pick just one book from the Old Testament.
My approach would be to look for the book that has the most messianic prophecies about Jesus. Perhaps Isaiah is one of the most important books in the Old Testament for this reason. And just think, he was the prophetic voice no one would listen to or heed.
Genesis is another extremely important book because it lays the foundations for the rest of the Bible. It helps us to understand God’s approach to be able to call his creation good again. It shows us the beginning of every part of his plan.
It would be hard for me to choose between these two books, and of all the other books that are in the Old Testament, each one of them contributes something to this salvation history narrative, fitting into the rest of the Bible beautifully.