Based on your question, I assume you are using the Kobo ebook reader app on your Galaxy S II.
The easiest way to load books to your SII is to install Mobilism Market on your phone;you can find it here: viewtopic.php?f=1&t=214777.
A second option is to find posts with QR codes and with the use of a barcode app on your phone download direct to the phone.
A third option you have is to download the books on a PC and then send the books via email to your phone.
Of course, there is always the old standby, transfer by USB from your PC.
If you think of using one of the first two option you will almost certainly need an app to decompress the files, such as AndroZip, which you can find in the Android section of Mobilism or Android Market.
Good Luck and happy reading.
btw: don't forget about Dropbox.
