Does it feel like there are a lot of series out there? It's soooo trendy to write/publish a series. I can see why. You invest all this time and effort into a writing a book (sizzle), promote it and the author (bang), and then it fizzles out. But, if you have a fantastic book, promote it while sizzling in the background, and have another BANG - you get more bang for your buck.
Here's another horrifying thought. You write a fantastic, over-the-moon novel and the publisher requests a second book, only YOU don't have one. That's what we call Shizzled. Ya got no sizzle and ya fizzled out.
So what do you do? How do you make a single book idea into a series? IF you are already doing a series, you're there. You've done your outline. You know that the red cat in chapter three of book one will be important in chapter 17 of book two AND the final scene of book three. If you're shizzled - read on.
Book one in a series usually deals with your mc "becoming" who or what they are intended to be. "What do you mean I'm an alien from Zordoc with mind reading powers and a tail?" How does your character cope with this and eventually accept themselves and/or the power they wield? Great book one. Bravo. Moving on to book two.
Book two in a series can do several different things. One is for the hero to find balance. Living in the world/finding happiness/finding love and etc and still being true to the person they became in book one.
Book two can reveal more of the world. Remember that awesome scene you had to take out of book one because it was too much information? I hope you held on to it because book two is the perfect place to fill us in.
Lastly, book two can show us what your mc does when the world falls down around them. Literally, have their world fall apart. Have the bad guy come out on top (Empire Strikes Back anyone?) and have your reader wonder how your mc will ever get to their happy place.
The key to a successful book two is to give your reader more of what they loved in the first book without repeating yourself - sounds easy right? (I heard you say, "Shizzle - no it ain't." To which I say, "Watch your grammar!")
I'll give you a movie example.
Hello, Spider Man.
In the first movie, Spider Man learns who he is. He learns that he has power/s. And, he learns that (say it with me now) "With great power comes great responsibility." Great first movie. The evolution of a man er spider man and his acceptance of his role in the world. Makes for a great story.
Second movie: Spider Man knows who he is. He knows what his job is. He just doesn't know how to be true to the love in his heart and still be the hero. Can he have both a life and a super job? Now this is good stuff. Why do we care? Because we all feel that. How do we balance our personal life with the outside world's demands on our time. Just because my demands come in the form of small children doesn't mean I don't feel that same pressure.
The second movie has all the elements we loved from the first movie. We have a handsome hero, the beautiful girl, the tech savvy villain who morphs himself into a monster, we have fight scenes, cool moves and the ability to fly through the city while taking out the low-grade thugs who get less than 30 seconds of film time.
Can you see how they didn't mess with the formula? People would have hated it if Spider Man were up against a group of mobsters from the 1950's. We want a cyborg, or a mutant by darn it and you had better give us one.
Don't mess with the formula.
What is your formula? Do you know? Can you identify it? More importantly, can you reproduce it? I'm sure, with all your creative abilities, you can. So go, write, but remember, "With great word power comes great writing responsibility."