Dear MiPE
Many thanks. Much appreciated.

I think I have reproduced this behavior, fixed as https://www.ventismedia.com/mantis/view.php?id=15696

That's correct. When I did an initial scan on install, I was able to see all my folder structure. And there was already a "collection" called "All Tracks.
Whenever there was a collection with the option of viewing "All Tracks", I could see them all.
However, I i went to a specific album - for example, E/Mediamonkey/A-C/Beatles/Hard Days Night/ - I wouldn't see the track list in the middle pane - nothing. If I went to All Tracks, selected them all, went into Properties and made sure I checked the box next to "Type" it would then re-scan them. Then I could see the tracks on an album by album basis.
Strange.
I have a good few thousand tracks and it only took a little while to make sure I grouped everything first, then selected all my Music, Classical Music, Audiobooks, Videos, etc and just went through the procedure for each group of tracks.

That's weird. So when you click on 'Audiobooks/Albums' and there select any album, tracks of the album did not show up in popup ?

Although...that being said, I do believe there is a bug. Basically, in order to see the tracks, I have to go into their properties, and even if they are already correctly labelled as "Music", "Audiobook", "Classical Music" etc, I have to select all the tracks of the same type, go into properties and make sure that the box next to type is checked. This forces the MM to rescan and add it to the database.
Otherwise, although I can see everything in "All Tracks" view, I cannot see the tracks in a single album unless I have done the check box thing.
It has taken a while...

Hi PeterCBR
Thank you so much!
Because the actual files were MP3 files and originally had the Type "Music", they didn't show up, because the Collection called Audiobooks had a criteria set that looked for the Type "AudioBook".
I changed the Type of all my audiobooks to match the Collection criteria, and Voila!!

Thanks again for your clear, systematic, logical solution. Much appreciated.

Check type of the tracks ? Most probably they have set Audiobooks (based on folder name from where they were scanned) ?

WOW!!
I DO see all the tracks if I use the "Entire Library/All Tracks" view. And they play fine.
So does this indicate an issue with the folder structure in my system?

So you see empty tracklist when you browse to 'Music/All Tracks' in media tree ?

I don't know if this is because of too many folders?
My structure is
E/Mediamonkey/audiobooks/author/collection/album/track

Mediamonkey won't show the files, even after a re-scan. The files are MP3 files and MP3 is included in options as a playable file.
The folders all show up, right up to the album. But no tracks are displayed.