The notebook function from previous games is now on Rosa's phone, along with her email and internet research capability, and is called "Compare Notes;" look for a piece of paper on the right of the Oogle icon.

The notebook serves two functions: it lists subjects Rosa can bring up when talking to others, and it can be used to combine two ideas to create a third one. For example, if you had "I'm thirsty" and "Water fountain" as subjects, combining them might result in "Get a drink." There are a couple of places in the game where you have to help Rosa draw these kinds of conclusions to move the action along; look here for help if you get stuck.