Well, I'm back up and functioning.
It ended up being a combination of answers that I found here and at the Sony reader site, because I had three problems: I'd lost the sidebar in the reader library, the reader library was very slow (sometimes locking up) and my reader wouldn't read EPUBs.
So here's what I ended up doing:
1. backed all my ebooks up (I have a separate file for the ones I purchase from sites other than Sony, so I had two locations to back up)
2. deleted my library as per Curtw in this thread (it's the fifth post down)
https://www.mobileread.com/forums/showthread.php?t=16796
3. Interesting note: After reinstalling, the library will reinstall all the books that you've purchased from the Sony/Borders store AND it reinstalls them all as EPUB. If there isn't an EPUB file for your purchases, there is a note that it will be available soon.
4. Then, before I loaded the rest of the library back in, I hooked up my 505, deleted everything on it, reformated the internal memory, made sure that the firmware was the latest version and then moved one EPUB file to it. Nothing - same issue, the reader said there was only one page EXCEPT this time when I tried to access that one page, it told me I didn't have access to the file. So I went searching again.
5. Adobe Digital Editions is REQUIRED for you to be able to read EPUB files on your reader (if I received that information, I missed it), so I downloaded the latest version from Adobe, realizing when it asked to remove a version that I had a old version of it on my system, installed it and . . . yep, I can read EPUBs on my reader now.
I did notice that the reader library works quite quickly after you reinstall, but once you hook the reader up the first time it slows down. Ah well, it's all working, I have readable files on my 505, so all's right with my world - back to my dissertation!