On iPad's iBooks, is there a way to sync and download all ebooks purchased elsewhere? I think on the iPad's iBooks, if we purchase an ebook, then in another iPhone, iPad, or iPod Touch, then that ebook is automatically downloaded into our bookshelf.

However, what if it is a .epub or .pdf purchased, say, from O'Reilly?  Sometimes the ebook is $29.99 on iBooks store but $25.99 on O'Reilly.  It seems that the book won't be automatically downloaded to the bookshelf, but we can use Dropbox or Box.net to transfer to iBooks, and all the highlighting and added notes will be there on each device (sync'ed).

But is there a way to automatically download all books to our library?  Otherwise if we have 12 books we need to manage 12 books on each of our devices.
iTunes is the solution that Apple offers to load content into iBooks if you don't want to perform that loading on the device itself.

I'm not aware of another product that does the same on a regular basis for easy set it and forget it syncing, but several others like PhoneView can read and write to the space iBooks uses to store content on iOS.