i use either the filament books
"personal content" page (i believe a similar service is available on the ETI site) which allows me to automatically convert a document into .imp format (if i'm feeling lazy), and transfer it that way, or else i use the GEB Librarian to send .imp files from my computer.
do a search for the Librarian on the site to learn more about it ; you can download a trial version for free. at the end of the trial it will still work if there are maximum 3 books in your bookshelf.
the other option is simply to transfer files to the memory card if you use one, but be careful with that, i've heard it's very easy to mess them up.