There are many book sequels and series on the shelves that are doing well: Twilight, Inheritance, Gossip Girls, The Vampire Diaries, Sisterhood of the Traveling Pants, The Keys to the Kingdom, Tom Swift, Gemma Doyle, Uglies, etc. A series is a set of stories that has no foreseeable end. Each story is pretty stand-alone, has the same core set of characters, and, when the book ends, the story ends too. If you read them out of order, they still make sense for the most part.
Sequels are different. Each book relies heavily on the previous installments, and it’s difficult to understand what’s going on if you pick up a book at random. Because of this, writing sequels, especially trilogies and such, is really tough. Essentially, you’re writing one HUGE story, and breaking it up into manageable chunks.
This also means that you have multiple storylines to manage: the bigger, overall storyline, plus each of the smaller storylines that create the larger one. If you want to write an effective trilogy, or quartet, or even a septet like Harry Potter, you need to know your overall storyline. And, you need to keep it consistent from beginning to end. If there are inconsistencies, readers will notice.
For example: THE SWEET FAR THING is the final book of the Gemma Doyle trilogy. A few things were revealed in this book that did not mesh with the previous two: Pippa's transformation, and Felicity's secret. Of the two, Felicity’s secret was the biggest shock. SPOILER WARNING: Not once was there anything to hint at the relationship between Pippa and Felicity in the first two books. In fact, they painted the opposite picture. Felicity sneaks around and constantly steals kisses from one of the gypsies. Plus, Pippa creates a gorgeous, fawning young prince when she’s in the realms, and she never grows tired of him. In fact, she chooses to stay with him rather than go back to the real world to be with Felicity. These kinds of things start the reader down a certain path with a certain frame of mind. So, when their relationship was revealed, it was jarring because it didn’t mesh with everything else we’d read.
A similar thing happens with Pippa’s transformation. In the first two books, it’s made clear that any human soul who stays in the realms too long will become corrupted. Then, suddenly, we’re told that they can choose not become corrupted. That Pippa has a choice. Here, the author has broken a rule that she established early on. SPOILER WARNING: In the end, Pippa becomes corrupted. But it’s because she chooses not to try, not because it was inevitable. If this is where the author wanted to take the story, then the souls-will-be-corrupted rule shouldn’t have been so absolute. At the very least, an uncorrupted soul could have been living in the realms, as proof that if Pippa had only tried, she could’ve remained herself.
While you’re keeping track of all these larger story ideas, you still have to keep track of the smaller ones, too. Each of these needs to have its own story arc, while keeping consistent with the larger story. A good example of this is the UGLIES trilogy by Scott Westerfeld. Each book, UGLIES, PRETTIES, and SPECIALS, is a story of its own. UGLIES is all about Tally being ugly. PRETTIES is all about Tally being pretty. SPECIALS is all about Tally being special. Yet it’s clear there’s more to these stories, that there’s a larger picture somewhere, which gets resolved in the final book.
I realize this is a lot of work. But if you let these details slide, you run the risk of alienating your readers. Both with this story, and with future ones. I, for one, don’t want to take that risk.