Hi cemerson, I recently bought a 505 and by the time it arrived I had a collection of about 60 ebooks ready for it.
I bought a couple of books, although most of the ones I downloaded were free, from this site and links I found on the wiki
https://wiki.mobileread.com/wiki/Free_eBooks . I used Adobe Digital Editions then, although I've since installed both the Sony software and calibre. I don't run both at the same time, and mainly use calibre now.
I've downloaded epub, lrf and pdf format books, although pdf isn't the best and if you can find a book in epub or lrf it's definitely better. I haven't needed to convert any yet and I've got a library of about 150 waiting for me. (I've only bought 5 the others were all free.)
Based on my experience, if you get a 505 you won't regret it.