A very small number of iTunes releases are accompanied by digital booklets. Essentially a PDF version of the CD booklet that can be accessed via a link in the library. An Apple Music subscription doesn't give you access to these when they exist, they can only be obtained when purchasing the album from the store. Most online album releases don't include them. I don't know if other streaming sites supply them.
If you've got a CD booklet with a purchase elsewhere (I think I've had a couple from Bandcamp purchases) it can be added to the library. The filename should be of the form Digital Booklet - <Artist>.pdf. When first added to the library it will show up under unknown artist unknown album. You can use Song Info (cmd+i) to set the album, album artist, artist, genre, year, etc. fields as applicable which should allow the booklet to show up as part of the album.
tt2