.. How do I use the notebook?

Rosa's notebook, which is located at the far left of the menu bar, isn't just a passive receptor of ideas. It's also a place where you can go to stimulate Rosa's thinking and draw conclusions about the things she sees. To do this, open the notebook (outside of a conversation), then click on the two ideas you want to associate. If Rosa finds any connection between the two items, she'll say so, and you'll have a new topic in the notebook.

There are a couple of places in the game where you can't progress until you help Rosa draw conclusions with the notebook. I'll tell you what to do in those spots.