Music library management

If you're an iTunes user, you may find that iTunes does not show album art for all of your music. A reason for this is that iTunes sources album art exclusively from the iTunes store. If your music isn't featured in the iTunes store, iTunes won't find the art. This is common for rarer releases, older releases, cover mounted CDs or bootlegs. It also affects music publishers who have not signed a deal with Apple, whether small (independent musicicans) or not so small (The Beatles!).

If art is missing, you need to either insert it manually (here's how) or use software like bliss to find the art and insert it for you. Here's a walkthrough of the latter.
First, here's our music in iTunes. We have missing art for "The Complete Collection" by Louis Armstrong and "Classical Tabla (Indian Drum) Interpretation" by Yusuf Mahmoud. Both albums were procured outside of the iTunes store and are not available within the store, and so iTunes does not have album art for them.
So we install and start bliss, and here's the settings we need. bliss chooses the default Windows music directory as a default, so we don't need to choose a different location. It's also important to embed the art so iTunes can find it later. Again, this is a default.
After applying the album art rule, bliss has located all the art and embedded it inside our music files.
Back in iTunes, we select all of our music... Either choose Edit > Select all or press [Ctrl] and [a] together.

Right click on all the selected albums and choose 'Get Info'.

Don't click any of the check boxes - just click 'OK'.

And finally we have our album art!

In the future, I'd like to streamline some of this in bliss. For instance, the latter stages where all of the albums are selected and you choose 'Get Info' could be avoided if bliss instructed iTunes it has updated the music files and to re-fetch the album art from the music files. If you want to see this kind of functionality, make a suggestion on the UserVoice pages.

That is how to use bliss to fix missing album art in iTunes.