I started a short story ("Kritarchy") today in addition to working on "Lived Too Long To Die".
But I have a good excuse: I can only visualize 10,000 words at a time.
On those days when I'm out in the world and working out of a notebook, it's a lot easier for me to keep the plot of a short story straight in my mind than it is to organize the multiple threads of a book plot.
This isn't a way for me to justify procrastinating on the book, it's just a statement of the current facts-on-the-ground. I'm also confident in my ability to juggle multiple plots, anyways, so I don't have to worry about getting confused.
I've discovered that writing a book is different from writing a short story. Writing a sentence or two in downtime throughout the day works for short stories, but a book requires concentrated effort of at least an hour in length, because I have to remember where it's going.
My theory is that this is all just a symptom of my having written about a hundred short stories that I like but only one book that I am barely satisfied with. I've got the "short story" mindset down, and I need to practice the "at-length" mindset. Presumably the only way to do that is to write more books.
I want to get better at writing Fantasy, by which I mean "Not Science Fiction per se". Yaknow, ghosts and magic and such. Or steam-punk. Or bio-punk. Or alternate history. Or some unholy union of all four, where the Nuclear-Powered Merrimac battles the Organic Regenerating Monitor while the opposing sorcerers Abraham Lincoln and Jefferson Davis have it out in the dream-realm of Robert E. Lee's conscience. And the ghost of Stonewall Jackson is the narrator.
I call it "Sibyl War".
500 words? Yes
Book "Lived Too Long To Die" - chapter 9
Short Story "Kritarchy" - in progress (~1/4)