In the adult business, acquiring editors are always asking what comes next? What do you have planned for book 2 and 3 and 4? It's funny really because most often, they don't care about anything but that first book. If it's not written to specifications, then they're not interested in anything else in the same series you might have planned. Yet if it's something they love, they want more of it. Because they know readers will too. And it's a way to brand an author.
So what if you wrote book 1, and 2, and 3, and didn't sell book 1? You have 3 books you can't sell. But if you sell the first, you've got a good chance at selling all 3!
The "what if" is the catcher.
I tend to write a story as a stand alone. Put my heart and soul into it. Try to sell it and while I'm trying, I'll begin work on the next book. But I'll make it a stand alone also. Why? Because if I don't sell the first book, I can't the second.
When I sold Heart of the Wolf, the adult werewolf book, I had no idea it would become a series. Yet here I am writing book 11 of the so far contracted 13 book series, and just finished book 1 of the new jaguar series, 2 more contracted. While I had shopped The Heart of the Wolf, I had started Destiny of the Wolf, thinking if the first one didn't go over, I'd have another one written that I could try out on editors and agents.
With my YA, I wrote The Dark Fae, sent off to a couple of agents, didn't get any kind of a response, so finally after several years thought I'd self publish. It was a one-book deal, although I did write another, stand alone also, though lost it in a computer crash, but I had no idea The Dark Fae would sell as well as it did. But it took off! The highest number of sales I made in one day was 134 on B&N!!! After making $7,000 that month and getting fan requests for more, I wrote The Deadly Fae and The Winged Fae. And have started on The Ancient Fae. Sales aren't anything like that one month, but they continue to be good!
For me, I start out with a stand alone title, and while I'm shopping it, I might write another or start another. Or I might work on something else to see if "that" book will be the one that catches the readers' hearts.
If you write series, or have thought about it, how would you/do you go about the process?