Among the easiest ways would be to install iFunBox on your regular computer:
www.i-funbox.com
Then, simply drag and drop all your books from iBooks folder and put them into Marvin's folder. It's easier and quicker than messing around with iTunes, although it's a similar procedure.
What I do: I put all my books into cloud software such as Dropbox or SugarSync, and then you can load them into Marvin (or any other e-reader) on any of your iDevices using the respective cloud app and the "Open in..." option. (More precisely, I let SugarSync mirror my Calibre folder, but using Calibre is not a necessity -- just an option.)
Emailing the books to yourself is another good option, but perhaps annoying if there are many books to email. (And I'm not sure if iBooks lets you email books from within iBooks; Marvin does allow emailing books from within Marvin.)