Well as far as I know, these books aren't officially the episodes 7, 8 and 9. What I think might happen is an official release of 3 books that could follow a slightly different plotline, and would be the first SW books not part of an "expanded universe", but part of the official thing. Those would be short books, about 150 pages each, so that it tells a story that would be possible to tell in a 2 hour movie with no loss to the story.