On the airplane heading to Stockholm, Joe is working on some kind of open notebook and he asks Joan (in kind of a needy way) if she wants to take a look at "this" (what he's been writing in the notebook). She says she'd rather keep looking at her crossword puzzle.
At the end of the movie after he has died, she's flying home next to her son and opens Joe's notebook. If you look closely, you can see that it's a list of ideas for a new book. "A man goes to Ireland and...", etc.
She turns the page of the notebook and affectionately rubs the completely empty page with her hand - and she smiles.
She has a blank slate to do whatever she wants with her writing now, I guess.