My story is way too long to fit into one book. Hence, I need to write two sequels. Now my question is, when I'm planning, summarizing, plot-creating, etc, etc, do I need to work on the stories of all three books or just the first one? Because I'm already having trouble figuring out the plot of the first one... So, is it necessary? Or can I first finish book 1 and then go to the others?