I just installed such a tag editor a few days ago, what a coincidence. I don't know what operating system you are using, but the one I chose is a graphical tool Kid3 . There is also commandline tool just called id3v2 . Both tools should be in the repositories in your distribution. However, these are Linux only I guess.

Kid3 is okay to me. You can get basic information from the filename into the tags, by specifying format variables like %{track}-%{title} in example and then click "to Tag 2" in example. Opposite is also possible, to rename all files based on the information from the tags. Nothing is changed physically on the files, until you hit the Save button.

Manual Edit: If you want edit a field with same entry for multiple files, then on the left side of the view with the files, just mult select with mouse while holding CTRL or SHIFT key. Then on the right side where the tag editor is, enable one of the entries like "Album" by clicking to show the tick. That means this Album entry will be updated for all selected files. Now type something into the Album field to fill or change it, enter and I guess that's it.

Import: I did not use this functionality yet, but it has multiple import resources from Amazon, MusicBrainz and other places. The Import options are in the top left menu under "File".