For Apple users, I recommend Ulysses. It’s a monthly or annual subscription, and everything you do on one device is available on the other. Ulysses is where I prepped last year. It’s also where I wrote the majority of what I’ve got down so far.

It’s easy export to DOCX, PDF or HTML for sharing and if you want to produce an ebook right then and there, it can do that too (haven’t personally tried that part, though).