It is not that modern books aren’t “good enough” to be in scripture, but that they aren’t “old enough.” Church leaders decided in the first several hundred years after Jesus’ lifetime which books should be included in the “canon” of the Bible and then they closed that canon. One of the criteria used to decide which books would make the cut was if the books were “ancient,” meaning written and handed down from our early Jewish heritage or in the first century after Christ’s life.
The teaching continues that since Jesus Christ was the invisible God made visible and the one who perfectly showed us who God is, we do not expect any new public revelation before his second coming. Since no one writing today has physically met the Jesus of history, no one can write a book that will add any new “revelation” that will be added to scripture. That said, the Church admits that “There is a growth in insight into the realities and words that are being passed on [in scripture].” (Vatican II, Dei Verbum, 8). So modern books that help us to understand our faith, lead us to further insights about God, or engender a deeper knowing of Jesus can all have a place in the mystery of God’s revelation.