You don't even need to load the books into calibre's database to view it. You can invoke 'ebook-viewer' from the command line directly with the epub file's path as the argument.
emacs can be a surprisingly comfortable text mode epub reader too, via nov.el¹ which has been discussed here². If you use a GUI emacs build you get inline images and other goodies, but starting emacs with -nw can be a reasonable solution for quickly checking a book in a term.
Note: You don't have to be a full-time emacs user to use nov.el.