The discussion on @thebibliosphere about content warnings in books made me go back to what Jason Porath did for @rejectedprincesses printed books. The above image is the content warning page describing maturity level in terms of PG through R, and then adds certain warnings like violence, rape, self-harm, etc.

This way, people could take these histories of women with cute illustrations and decide which they could read—for themselves and also young readers interested in the art and history. It works well on the various entries as a quick guide, and doesn’t detract from the actual content.

More like this in book publishing would be amazing.