Well, you're almost to the end. Congratulations. It seems as if I'm not the only one doing this sort of thing, but I hope my contribution is good. If not...well, too bad. I'm writing it anyway.
The penultimate novel is a fragile thing. This could be lumped in with the "sequel post", for it can be the second book in a series or the eleventh (*coughcoughWheelofTimecough*). But this one's important. This is one where a cliffhanger seems a must. Look at The Rise of the Wyrm Lord, The Battle of the Labyrinth, Harry Potter and the Half-Blood Prince, PENDRAGON: Raven Rise, Gregor and the Marks of Secret, and many others. Sure, some books don't have major cliffhangers - i.e. Brisingr, The Well of Ascension (sort of), and Charlie Bone and the Shadow. But a really good cliffhanger will keep readers waiting with bated breath the entire span of time until the finale is published.
And... that's pretty much all I need to add. Oh, and here's this.
Writing a Prequel
The prequel is even more fragile than the penultimate novel. It doesn't just need to be good, it needs to be amazing, or else the readers won't care. Actually, on second thought, just don't write a prequel. Please, for the good of mankind.