I use the mobile app primarily and excessively tag my books, which ends up being super tedious when going back and tagging books that I read pre-this feature. I would love to be able to add books from a tag’s page, like adding books to a collection on Kindle or something. For me, it would be most useful to have a list of all of my read books and then be able to check off all of the books I want to apply the tag to, and then a save or apply button.