UPDATE: It wasn't the software. Sony Support says that the 300 software won't work on the 505.
But they recommended several things, one of which worked - a hard reset.
http://www.kb.sony.com/selfservice/d...rnalId=C441368
I made sure that I had everything copied off the 505, then did a hard reset which solved the problem with Library. Then I went into the Store, reauthorized the 505 - which required that I reauthorize with ADE as well.
As long as I was messing around, I pointed my calibre library at the location of the Sony book files. Then I put an alias to that file on my desktop, and one in my download folder, so that from now on, all my books, articles &c are going into the same folder. It will be interesting to see what calibre & Library do with them.