Originally Posted by Jabby
I added a "read" tag and thought the right click on tag and then hide would do it. Wrong!
Left-click the tag once. You will see a green + next to the tag, and the library will show only those books with the "read" tag. Left-click the tag again. You will now see a red - next to the tag, and the library will show only books that do not have the "read" tag.
After clicking, look at the search box. You will see the search that calibre constructed. As Hitech_luddite pointed out, you can save either search as a saved search, making applying the search faster.
If you make a saved search, then you can use the 'Restrict to' box to create a temporary virtual library of books matching a given saved search. For example, if you create a saved search for books without the "read" tag, then you can create a virtual library of books you have not read. The tags, authors, etc will adapt, showing you only those items in the virtual library. Searches work on the virtual library. Etc.