In most programs there are global Mac services available in the Services menu. For example, if I select text in Chrome I see the usual service options. They allow me to do a lot of things with the selected text, move it to various applications and destinations, etc:
But if I do the same in Calibre Viewer most of these options are nonexistent except a few developer services.
My guess is for some reason Calibre is not making the text presence aware to the OS but I'm not really sure how this works on mac.
It would be nice if text services could work with Calibre Viewer. You can see some useful applications for this in my first picture, like summarization services, calculating statistics, shuttling text to evernote/text editors/email/journal/other apps, doing calculations in math/engineering texts, etc and being able to assign custom shortcuts to any of these.