I'm a very linear writer. I know that some authors write their books as individual scenes, possibly out of narrative order, and then string them together to make a book. That's almost impossible for me to imagine. When I write a book, I start at the beginning and move step by step to the end. I'll often have fairly clear ideas about how the story will develop - I may even have a scene list - but at least ninety percent of the time the sequence in which I write chapters is the sequence in which they'll appear in the final book.
When it comes to multiple books, I'm the same way - linear rather than multifurcated. Many of my author friends work on several projects simultaneously. They tell me that this allows them to switch to a different story when they get stuck on the current one. This makes sense - but I can't seem to do it. My natural mode of operation seems to be one project at a time. Occasionally I'll interrupt work on a longer book to bang out a short story for an anthology that has an urgent deadline, but I've never had two active book-length projects going on at the same time.
This is actually rather limiting. Because it's not at all uncommon for me to get ideas for new stories while I'm in the midst of my current WIP. I'll get all excited - but I can't quite bring myself to halt work on the story at hand in order to dive into my latest idea. I've learned over time that in this circumstance, it's critical for me to make detailed notes about the new idea, while it's fresh in my mind. This often includes snippets of dialogue or an opening line. Then I'll put the idea aside until I'm done with the book that's my current focus.
Sometimes the new ideas grow cold by the time I get around to trying them out. Sometimes there may be a gap of years between the time I first come up with a story premise and the time I start working on it. I have to live with these consequences.
Maybe some authors who are reading this envy me. I have friends who complain about having too many ideas, too many concurrent projects, so that they can't focus on any of them. I can see that this would be a problem - a different sort from mine.
In any case, I've learned that you can't really fight your own natural approach to writing. That's one reason why new authors shouldn't pay too much attention when someone tells you how you should approach the process. You have to find what works for you. And you may find that your process changes over time. However, you really can't force it - or at least, that's what I believe.
So for now, I'll just have to be content with my own obsessive-compulsive approach to writing and produce one story at a time. I just finished an erotic noir novella and sent it off to my crit partner. Now I've started working on my multi-genre extravaganza Rajastani Moon - my attempt to write a book that incorporates steampunk, Bollywood, menage, shapeshifter, Rubenesque and BDSM elements - and maybe I forgot one or two genres there...! I'm having great fun (it's a huge relief after the darkness of Bangkok Noir).
And what comes after that? I'm not 100% sure but I have an idea for a new BDSM novel. I don't want to plan too far ahead, though. Right now, my plate is full!