Keynote is great, but it's still slideware. People need to think about the iPad as a printer for dynamic content. It's smart paper.

Edit: Thanks to gte910h for pointing out iBooks Author. I wasn't aware of it!

http://www.apple.com/ibooks-author/

You're thinking of the iBooks app (seriously, check out the free apple iBook creator). It's very dynamic content.

For publications you will use locally on the iPad, maybe.

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.

Not the book, the output of the program. You'd need another tool anyhow because of the extensions.

Yes but the output format used is proprietary superset of the ePub standard so it wouldn't be possible to view them on other platforms anyway.

Time to bring back Hypercard? : P

Web standards are probably your best bet as a smart paper API.