Here are some great books you can run reading groups on:
You can also run reading groups about books that are on object-level topics that members of your group might be interested in.
Reading groups are a great way to dive into forecasting, but they can sometimes be difficult to run well. Here are a few guides on running high quality reading groups (note: some of these are geared toward Effective Altruism clubs, so make sure to adjust accordingly):
You probably want members of your club to spend a minority of their total time in your club on reading groups. Their time should probably be primarily spent on doing the thing, rather than sitting in a group reading about how to do the thing.