I think you're misguided here, the most difficult to handle truth has to be written in a book. Movies are not good at explaining real things because they focus on short snippets of life, and direct knowledge do not scale.

For example truth about something like cultural revolution in China or religious wars in Europe is likely best conveyed in books.

Maybe when a truth is so bad it cannot be written then it has to be sung, or hummed, and music would be the deepest and most reptilian way to keep memories.

Amen to that! The other thing is that books don't have to teach you anything. it might just be to disconnect, open up your mind, see something in a different way. I would not necessarily call that "teaching". It can help with creativity tremendously! But then again, not every one can't handle creativity!