Slouching Towards Bethlehem

Didion distinguishes the two in this way; a notebook is used to "note" observations, reminders, so to speak. Whereas, a diary, distinguishes what really happened from what might have happened.

Didion uses her notebook to record oberservations, snippets of dialogue she hears along the way, impressions she's garnered of people and places, and a correlation of memories inspired by these things.