We have a rule where you can’t talk about the book club book until the day of the meeting. In the chat specifically, but I enforce it when I’m with people and they start to bring it up in person (I lead). I sort of frame it like fight club. You can’t talk about it.

Since you’re trying to make friends maybe suggest that this line of thinking is still a spoiler for you and gives you pre conceived ideas about the book. Nicely suggest any thoughts on the book be held until the meeting day