An introduction to computer audio
Normally your media player will supply the relevant information for the tags using an internet database.
But sometimes it fails you. There are a couple of alternatives.
A Google image search often retrieves missing cover art.
A lot of information about composers and performers often including a discography can be found here.
A well-structured source of information.
Good source for classical music.
If you look up Beethoven you get over 700 entries including all WoO (Werke ohne Opuszahl).
Lots of information about composers (7000) and their works (55.000) in the Petrucci Music Library.
You can even search by melody!
Now if I would have know how to play the keyboard....
Longbored Surfer took the trouble to publish all Billboard top 100 charts from 1946 on.