This phrase has been used in certain circles to describe a religion whose faith is centered on written scriptures. While the Catholic Church possesses written scriptures, the description "people of the book" does not really apply. Rather, the teaching authority or magisterium is a living thing composed of the bishops teaching in union with the pope. As such it is the living discernment of this magisterium that tells us which writings are scripture in the first place.
There has been much coverage of the SSPX in this blog so it is important to discern what this group really is. They are people of the book in that the magisterium they refer to is not the living teaching authority of the Catholic Church which they have alienated themselves from. Rather it is the texts and decrees of past councils, popes and church fathers. In a sense, when they left they took their library with them. In this they have far more in common with Martin Luther than the Catholic Church.
When the Augustinian friar decided to make his break with the Catholic Church, he had to appeal to an authority he could claim was over and above the one he was rejecting. This was the birth of the doctrine of sola scriptura. Men could read the book and decide for themselves what it meant. The rebels at Econe took a somewhat bigger library with them but the principle is the same. They truly are people of the book.
Now this is where it becomes somewhat muddy. The SSPX are protestants but they are protestants with a difference. They mirror most of the Catholic Church's structures and doctrines and they even have their own hierarchy. Their liturgies are the envy of many who find their own parish mass unsatisfying. There has been much traffic back and forth due to the uncertainty of their status. The pope has decided that this uncertainty cannot be allowed to continue.
Why the obsession with the SSPX here, then? I think the answer lies with the struggles of the advocates of the tridentine mass over the years. They are a beleaguered community which has had to struggle with opposition and misunderstanding. There is a hope that an influx of new blood from the SSPX and a regularization of their status will invigorate that community. I pray for the success of the current efforts at reconciliation.