there is two ways you can add other books to your sony reader (that i have found)
1. when your in the sony library software on the left is a list which is your library, books etc. the bottom one is computer. that is your computer and you can open that and look through the files as you normally would until you get to the file that holds your ebooks. then just drag and drop them into your book libary and copy them on to your reader like your normally would.
2. and this is way easier get an sd card put your ebooks on it and put it into your sony reader. yeah it really is that simple you don't even have to use the sony software.
hope that helps if not send me an email or pm and i will talk you through it, i've just updated my sony software so i can (no i don't use it and haven't for months)