"When do you know when you're ready to write a book?"
Most new writers, I think, start with short pieces--essays, articles, short stories. There's the gratification of starting and finishing. Jumping into a book length manuscript takes a ton of commitment and new writers don't know how committed they are, generally. It's a little bit like running: Before you run a marathon, you will run around your neighborhood, run longer routes, and get in shape before you attempt a marathon.
You may find, in writing short pieces, that you want to say more, that short pieces are, well, too short. That's when you know.
Or you have a burning passion to tell a story that needs more room than a short piece can offer.
What about you, out there? When did you know it was time? Or do you remain loyal to the short form?