This is more of a Related Tools subforum question, I think.
And in fact, there is a sticky there for:
Minimalist Windows App for Calibre
It should act as a frontend for calibre's database and launch books via whatever configured reader you have.
calibre could do that also if you go to Preferences ==> Behavior and uncheck formats that you do not want calibre's ebook-viewer to handle. However, Marty_B's app should be more tablet-friendly.
Hope this helps. It isn't exactly what you asked for (a way to add books en masse to a reading app) but it does perform the same general function in a different way -- kind of like Calibre Companion for Android.