I like to open new ebooks directly with calibre's
ebook-viewer.app (found inside the
calibre.app) so they are not copied and added to the calibre library, but since macos High Sierra I get the error:
"ebook-viewer cannot be opened because of a problem".
My workaround is to run the ebook-viewer code (found inside the ebook-viewer.app) from the command line with the currently selected ebook filelpath as an argument. I created an ActionScript App that you can make the default "open" application for any particular ebook type (.epub in my case). It only opens one book at a time. Here's the ActionScript:
