I just started the Calibre Library on my workstation.
Went: Connect/share -> Start Content Server.
If I go back to "Connect/share" I see:
"Stop Content Server [192.168.0.130:8080]"
So 8080 is the default.
Connecting with Firefox localhost:8080 is fine.
So I went to Pi and paused the Wizard of Oz video

Didn't work! Ah, but it helps to connect the LAN cable as I never setup WiFi. Now works.
So started Calibre, then content server on Pi (only one ebook rather 7000+ on workstation). localhost:8080 is OK
Tried 192.168.0.170:8080 on Workstation. Works. Though that IP of the Pi isn't fixed.