Justine writes awesomely about the trials and tribulations of the third books of trilogies - the fits and starts, the rock solid ending that morphs into something, the difficulty of publishing as you go as opposed to writing all three before selling them. I luckily have been blessed with the opportunity to write all three before publishing them, because I cannot sell the first for the life of me. But actually this has worked out to my - the story's - advantage. Writing the second book had a ripple effect on the first. Starting the third had an effect on the second and the first and so you go back to make it all work - Justine says she did it six times - I lost count - and this would have been impossible if I had sold one of these books before the others were complete. Which they're not. I am yet again revising the first book. I am halfway through a revision of the second and I have a hundred pages of the third. No doubt there will be more changes and revisioning as I go.
She also links to a post from Diana who comments thoughtfully (and awesomely) on the best sequel of all time: The Empire Strikes Back. The second book/film is always the dealbreaker. Either it's Empire or it's The Matrix Reloaded. I don't think of the second and third books of my trilogy as sequels; they're units of a whole. But at the same time, they're individual novels. They have to succeed on their own. They have their own internal story. Each has a beginning, middle, and end (to one extent or another) but each is a vital and necessary component of a much larger whole. The second book in particular has the hardest job because it has to echo the first, but not mimic it; it has to advance the story, but not complete it; and it has to deepen and expand a narrative without tearing it from its roots.
I kind of dig my second book. It's the third one that gives me fits. I know how it ends - I have for years now - but I don't know really the details and won't until I'm there writing it. It evolves all the time anyways, just like the whole thing. I just want it to be over! I never want it to end! Arrrrrgh!