Yes. See
Command Line Interface / ebook-viewer.
You can make the calibre viewer the default for opening an ebook format. How you do this depends on what operating system you are using. On windows you right-click on the ebook file, choose "Open with", Choose another app, select the calibre ebook viewer, and check the box "Always use this app to open ...".
On Windows you can create a shortcut to ebook-viewer, for example on your desktop, then drag ebook files onto the shortcut to open them.
And so on ...