Thanks for your prompt reply, Kovid. I really appreciate how much of your time you dedicate to supporting Calibre. Your suggestion about trying a single book gave me a few ideas to try this evening.
I'm using version 4.3, official binary.
I did try a couple of single books. One was a standard .epub file from my collection, another was a file I quickly made with
Each time, the command exited with "Not Found". Deliberately mistyping the file path exits with printing the entire path with lowercase "not found".
I tried running tail -f on my calibre log file and nothing is being logged to the file when I try - what seems to be - a properly formatted command. When running the calibredb add command with improper login credentials, it does show failed login attempts, so that validates at least my logfile is being written to properly and my host address is right.
Hope this helps in guiding me toward more things to try. Thank you again!