Dropbox can be used to sync files over multiple devices just to get the files on the device. You would have to share the account though or generate a new account just for this purpose. Once the files are within Dropbox's folder on the device you can move them over to a folder out of Dropbox's control.
With dropbox you are not limited to just e-books. You can share music, pictures, apps etc. with anyone that has access to the account. The sharing can go both ways as the common denominator is your Dropbox cloud storage.