If you're intending to publish via Apple, I'd say wait. After two weeks of waiting for Apple to approve a book I yanked it and am having the manuscript converted for Kindle sale as we speak. The conversion from the .ibooks file to anything usable in the real world was a giant pain. All of which was predictable and I knew that going in.

But as a book-writing tool it is great. I wrote an entire book on a direct flight from Los Angeles to Dubai. Edited it on the way back. Way, way better than Word.

Now I'm using Scrivener.