In most editions of the Christian Bible, Genesis comes first. But it was almost certainly not the first book of the Bible to be completed.
Biblical scholars date the composition of the Book of Amos to 750 BCE, around the reign of the thirteenth king of ancient Israel (Jereboam II). In contrast, Genesis is thought to have reached the form we know today around the 6th century BCE.
Of course, like many early biblical texts, traces of earlier texts are apparent – in this case, the Sumerian “city laments” that mourned the fall of ancient cities. The Book of Amos may have been influenced by the Lament for Ur, which was written about 1250 years earlier. That lament is also thought to have contributed to Lamentations, Jeremiah, Ezekiel, and one of the Psalms.