Yes, I know that it sounds rather odd to think about an app when you talk about writing or publishing, but I can tell you that this app has changed the way that I do all of this.
What is it?
It's DropBox. What does DropBox do? It allows you to sync information across devices. So, for example, I am currently working on a book that makes great use of a database. In my DropBox folder, I have deposited both the front matter (the text) and the database. I have DropBox loaded on my iPad 2, on my netbook, on my Android phone, on my computer at home, and on my computer at the office.
From any of those devices, I can access my files for the book. Any changes I make on any of those devices syncs out to all of them as soon as I log on to that particular device.
In addition, it's an easy way to create a back-up. Once you have dropped your files into the DropBox folder, those files actually live in the cloud.
One thing to remember, though, is that when you are working on your book, make sure you are using the version in the DropBox. It does not sync to your desktop. You want to make sure that you are using the most-up-date version.