stevejay:
For books elsewhere, did visit Baen's website yet? Lots of free and reasonably priced SciFi there. Of course FeedBooks and Project Gutenberg are good source for older books now in the public domain all worth a read/re-read. I found almost all the H. Beam Piper stuff on PG, I forget if FB had them as well or each had a few. Also found the whole Barsoom Series by Edgar Rice Burroughs on one of those two sites. I think you can also find them here.
To help manage content format look at the Calibre forum to find a nice bit of software to manage your library, transfer books to your reader and change the format. In fact that whole
E-Book Software Section will help as you will eventually want to try different formats or need to change the format of a book to your preferred format.
Sounds like you are on your way though...but know that for pretty much any non-drm ebook, there is a program to convert it to something your Sony can handle. So, I would look for titles then follow the suggestions from one of the ebook software forums here on MR.