The first question to ask yourself is: “why do I need to say anything at all?”. If you don’t like the book or think it’s garbage, you don’t need to say anything. It’s not your job to educate your boss on what’s good or bad. So keep your yap shut.
The second issue is how to feign interest or how to steer the conversation. I would treat something like this the same way I treat a conversation about religion, race, or gender, that might disagree with amongst colleagues or people I don’t know.
As others have said, you can turn questions around and ask them. “It’s not my type of book but did you enjoy it? What part did you like?”
The key to it is to leave your ego behind. If a child comes up to me and says they liked some trite novel, I wouldn’t disparage them. I’d feign interest and ask them to talk about it.
The fact that you talk about “redline the shit out of it” makes me think it’s your ego that’s the problem. You think it’s your job to correct your boss and tell them why they don’t understand good writing. That’s an ego thing.