[razmichael]

[EDIT]: Solved the 'issue'. As the current crontab (runs every 3 minutes) checks if the GUI is open and does not even try to run calibredb to add any files found in the autoadd folder, this can continue to run (without trying to add anything) even when the GUI is open. By setting up the GUI to use the same autoadd folder it means that either way (no gui, running silent/headless, VNC window and running the GUI ...), any books dropped in the folder will be added - just by one or the other mechanisms. So basically a seamless adding of books from any authorized device.[End EDIT]

Thanks for the response. The goal is to rarely have the GUI up unless really needed (VNC into the Pi) while the scripting along with calibredb will add any books in the background that are dropped in this network shared directory/folder. As it looks like there is no way to "tell" calibredb to ignore the "another calibre program is..." warning and continue to add books (if found) I'll just let the background process continue but it will skip any new books until the GUI is closed (what I do now). Other than this minor limitation, the process works great anyway - put a book in the folder from anywhere conputers, phone, outside the home network for devices with authorization) and a few minutes later it is readable with the web interface.

Thanks for all your work with Calibre.