One problem with trying to write a part-epistolary novel is that every time you start a new missive, that stupid little assistant shows up and says "It looks like you're writing a letter. Do you need help?"
No, you stupid piece of silicon garbage, I don't. If I type "Dear John" at the top of a new document, it might be a letter. If I type "Dear John" at the bottom of a 6,000 word document, it's probably not going to be a letter.
At least the assistant I have to deal with is the annoyingly cute Mini-Mac, not the simply annoying paperclip...
I prefer Rocky, the brown cartoon dog.
hey! you'll make it cry!
One word, one app:
LaTeX
(:p Although in all honesty, it would probably be harder to write in TeX than in anything else...)