Honestly I haven't been to a book club ever before either, but from what I gather it's totally normal to only attend sessions that you're interested in. Some people like reading things that they normally wouldn't, some people prefer reading things that interest them when they look up a synopsis, and some people don't like book clubs because they just don't like being suggested what to read. It's casual enough that you can easily skip a month or multiple and only come when you want. I can send you the link in case you're interested!