"Not in my library" doesn't correspond to reality?

I recreated the library. The recreated library creates a new file for media content, and finds nothing there, so that allows me to re-download the missing content.

That's good right there. I was afraid I'd lost my content. Thanks.

Different matter that I'm going to have to reorganize my files manually. What's odd is that the Purchased items in iTunes are listed by Artist, not album, and I mean, every artist that took part in an album project. There's an entry for Abbott, and a separate one for Costello, even though they are both on the same album. Names I don't recognize, turns out are Handel's Messiah soloists listed separately. Anyway, I'm re-downloading, and will sort out the files later. Then, presumably, I'll have to Add the files back into a Library.

There's no macro that scans a given directory for Media files and automatically puts them into the iTunes library?

Yes, that's what's happening. I opened a new library because I was nervous about deleting anything in the old library as a way to force a new download.

What's odd about the Purchased list is that it goes by Artist, while the library folders go by Album. So for example I have three separate entries under Purchased for Linda Ronstadt, Little Feat, and Little Feat (with Linda Ronstadt). When you click on any of the three of these Purchased entries, it points to the same album (i.e., folder in the iTunes music folder). This makes it hard to compare Purchased to actual to see what's missing.

The other thing I notice is, you need to check that iTunes actually downloads everything in the album. Pressing Download All doesn't do any good. It generally misses a lot of tracks, and you have to go back and re-download, or download specific individual missing tracks.

Anyway, I understand better now that the library files are not looking into the Media folders. I need to get the Library aligned with the content, and iTunes does not do this automatically/reliably, so I'll need to double check it.