Shawn Blanc writing for The Sweet Setup:
Ulysses is the central spot for where I store all my notes, research, and other tidbits of inspiration. It’s also where I toss all my writing ideas, and it’s where I actually do all my writing.
What’s so awesome about Ulysses is that it excells at each of these functions. Not only is it ideal for capturing your ideas it’s also the best app there is for doing the writing. Win win!
I also use Ulysses for all my client writing and all my blogging. I haven't got into it for by books but you could do that. I grabbed a few of the links Shawn provides for further reading.
See here I am using it.