And that won't help you get around the fact that DRM is security through obscurity. If DRM were cryptographically secure, we wouldn't be able to read the books

And that won't help you get around the fact that DRM is security through obscurity. If DRM were cryptographically secure, we wouldn't be able to read the books at all. The fundamental difference from PKI is that "attackers" have both the content and decryption keys on hand. If your device/app can decrypt a given book in order to display it, then you can be sure that someone else will be able to emulate your decryption method, or just rip the decrypted book from memory. Remember that DRM is not about protecting content over the wire. It is about protecting content from "malicious" users, who also need to be able to consume that same copy of content. If you can write an encryption scheme that does not require consumers to store a key, but still can only be read by a given consumer, you can write your own ticket.