I have successfully upgraded ADE on 2 computers but on the 3rd the library does not migrate & ADE 3 is not uninstalled.
Check if found the missing eBooks in the My Digital Edition Folder under My Documents.
The books were not missing. The problem I asked about was - Why did the installation program not offer to migrate the library, and then offer to uninstall ADE 3, as had happened automatically on the two previous updates I had performed? I had to manually uninstall the old program & then manually add all 276 books to the new installation.
Normal Digital Edition used the same folder and you get display the eBooks, but sometimes is not work.