I use save to disc in Calibre, and save the books to a folder that is shared over wifi.
Then I use the app FolderSync to sync a folder on my Xoom with the folder on the share.
Since my Xoom can't write to the external sd card, I have to use the internal card, but you might be able to use FolderSync to write to your external card?
I have FolderSync set to check if there are any changes on the share every hour, if I am charging my Xoom. So the sync usually happens without me having to do anything.