Thanks for the advice.
Since the sysfs was the standard Linux version and the Sony PRS-950 were both otherwise in decent shape, I decided that the problem was probably a stuck USB bus.
I confirmed this by trying to hotplug USB flash drives, which would not mount even though they were detected by udev.
I rebooted my Linux box. When I hotplugged the reader, udev correctly created a "READER" directory and mounted the reader; and Calibre recognized the device, which is now working properly.