Hello Carmine,
congratulations for the book, it is an invaluable resource!

I’d like to share an issue happened to me trying to debug two boards simultaneously, as described in the paragraph 23.6.
If I copy the serial number in plain ASCII characters as you wrote, the ST-LINK is not recognized. I found a useful answer on StackOverflow that resolved the issue:

http://stackoverflow.com/questions/29121050/openocd-debugging-multiple-devices-at-once

Basically the plain ASCII characters are not accepted, I had to insert the serial number in hexadecimal format, copying it from the log file.
Furthermore, due to the fact that the serial number now is composed of “strange” characters, the command “hla_serial xxxx” inserted in the “External Tools configuration” page is not working, and Eclipse gives an error. I resolved the issue by putting the command in a custom .cfg file (as explained in StackOverflow).

Alessandro