Scrivener is perfection. Seriously :)
It not only has a great writing environment and structural tools, but it also gives you everything you need for exporting your work.
I'm designing eBooks mostly and have not found a tool that handles file types (epub, mobi, Kindle etc.) so freaking well! And trust me - I've searched a lot!
Scrivener is worth every penny, because it will save you lots!
RUEHL Publishing House