Does one tag only correspond to one notebook? Because when I use that tag for the next book, I don’t see it tagged. I hope the developer can design more features for creating folders to group many similar notebooks into one folder or arrange notebooks in a list form to make it less confusing.
You can give the same tag (or tags) to as many notebooks as you like. Tapping that tag will filter your shelf so that only notebooks you’ve given the tag to will be shown. Here’s a little screen recording that might help.