Thanks for the help, AnemicOak.
Short story: I got it working and can read my ebooks.
Long story:
I have my old version of Nook for PC still on my Windows 7 laptop, but it only opens the one book I ever downloaded. I couldn't find a way out of that book to any kind of library userface. Probably a lack of tech savvy on my part. But I downloaded a new version onto my 8.1 laptop and saw my whole library, opened a book I hadn't previously read, etc. I look forward to finding some time to read them.
I still haven't looked into what it will take to get them on my Kindle, but since I'm not talking about that many books, I may be satisfied with reading them on my PC.
Is it just my trouble with the Nook clouding my understanding, or can ebooks not easily be "owned" (i.e. kept as files on personal hard drives)? I'd like to figure this out for my Amazon purchases, mostly.