It's downloadable from the Sony web-site.
(long link follows, starting with http: and ending with Id=-1)
http://www.sonystyle.com/webapp/wcs/...0551&langId=-1
Just click on the downloads/drivers in the Technical Support section, and then enter PRS-500 or PRS-505 for your model and then follow prompts to enter your operating system and then download the software.
Or do a site search for ebook library software.