First of all only look up legal free ebook sites because if you take streamlined ones it is stealing, violation of copyright laws and the authors should get paid.

Whatever reader you have or are using should have a program.  I too have the sony prs-505.  Love it.  You install the program once and it is exactly like itunes.  After the one time install.  You just click on the ebook icon on your desktop, click on store, look at the books coveres and descriptions and price (search by author, category, price, sale, bundles, etc.), add to cart, purchase, connect the reader to the usb and your computer, then drag the books you bought to the reader and done in a few minutes.  The book supports some other formats like PDF so I can get free ebooks in that format.  I download it to my books folder that comes with your computer.  Then you just transfer it to your sony library and drag it to the reader.  Super easy.  Just make sure not to steal books.  God sees everything.