I've not written a book manuscript but do a lot of writing, reports mainly. I use Pages and some other writing apps which I have found on the App Store. There are various programmes to suit the kind of screen layout that you like so it's worthwhile trying a few to see what works best.
I'm not the worlds best typist, by a long way, so I find that the screen keyboard doesn't work for me for long periods of typing so I use an Apple Wireless Keyboard for heavy typing sessions.
The Archangel