The Old Testament books were compiled by the Jews and were part of the sacred cannon of Scripture hundreds of years before Christ.  The New Testament books were written and preserved by the early church.  Special meetings of the church were called (church councils) to officially ratify the books that were accepted as inspired.  This was necessary because spurious books were circulating that were not of apostolic origin.  Interestingly, nearly 100% of the New Testament we have can be found in the writings of the early church fathers.