Thus it fills in the gaps in the story, as the previous five books only reveals what Bernard can see and think he understands. It also tells the back story leading up to the story in the five novels, which has only been hinted at previously.
This fills in the missing pieces of the narrative, as the previous five books only present what Bernard can see and think he comprehends. It also provides background information leading up to the story in the five novels, which has been alluded to previously.