I've never been the type of writer who can work on more than one book at once. When I'm writing a novel, I'm usually so focused on it that I can't think of other novels for fear that it will pollute the one I'm working on. That changed recently when I stopped working on the YA fantasy series that occupied the first half of this year.
Last month, I started a new novel outside the YA genre. It's a thriller because thrillers are my second favorite genre after YA. I spent a couple of weeks coming up with the outline for my new book. The exercise re-ignited my writing passion, and I was eager to work on it. I wrote the first chapter, and writing became fun again.
Then the workload at my day job got out of hand. For two weeks, I had no time to write or to even think about writing. The thriller was in the back of my mind, but I had pushed it far enough back that it sat with the YA series I had abandoned. When I finally found a little bit of time to write again, to my surprise, I wanted to continue with the YA story instead. So I did. I didn't force myself to plow ahead with the thriller. I let my muse (or whatever it is that drives me to write) lead me back to the story that I thought I had left behind.
Now I'm going to work on both books at once. When I find the time to write, I'll let my inspiration dictate which story I want to work on. It'll take me longer to finish either story, but I'd rather continue making progress, albeit slow progress, on two stories at once than to not make any progress at all.