There is one final point, the point that separates a true multivolume work from a short story, a novel, or a series. The ending of the final volume should leave the reader with the feeling that he has gone through the defining circumstances of Main Character's life. The leading character in a series can wander off into another book and a new adventure better even than this one. Main Character cannot, at the end of your multivolume work. (Or at least, it should seem so.) His life may continue, and in most cases it will. He may or may not live happily ever after. But the problems he will face in the future will not be as important to him or to us, nor the summers as golden.