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RobSys32 - if you only want to refer to other libraries - i.e. read metadata and content, you could use the
CalibreSpy plugin. You would then use your OS to switch from the main library GUI, into different instances of CalibreSpy that you would have started already. On Windows I use Alt+Tab and sometimes Win+Tab. The switching is fast because the databases are already loaded into the different instances of CalibreSpy.