I'm happy and relieved to report that a lengthy phone session with BN tech support seems to have resolved the problem. Whew!
The support rep was initially stumped, when after two hard resets and waiting awhile (5-10 minutes) after each the Reader still did not work, and no NOOK books appeared in my NOOK library. (She said the problem was unusual, but based on my online research, it's actually not uncommon. I hope others who encounter the same problem can benefit from this post.) After performing a third hard reset, the rep said she was going to disconnect, and she advised me to wait another five to ten minutes to see whether or not the NOOK books appeared and the Reader worked. If those desired outcomes didn't occur, she said to phone back, and they'd make arrangements to send me a replacement device.
Fortunately, the third time was the charm. About 15 minutes beyond the call's conclusion, the problem seems to have resolved itself. My NOOK books have appeared in my device library, and Reader reads NOOK books and third-party content alike.