I see the iOS version of the very popular writing software package Scrivener is now available.

It looks like you need to updated version (2.8) of the desktop version for synching.

Tailor-made for creating long manuscripts, Scrivener banishes page fright by allowing you to compose your text in any order, in sections as large or small as you like. Got a great idea but don’t know where it fits? Write when inspiration strikes and find its place later. Grow your manuscript organically, idea by idea. Whether you plan or plunge, Scrivener works your way: meticulously outline every last detail first, or hammer out a complete draft and restructure later. Or do a bit of both. All text sections in Scrivener are fully integrated with its outlining tools, so working with an overview of your manuscript is only ever a click away, and turning Chapter Four into Chapter One is as simple as drag and drop.