He sighs.
-- Merel probably knows what she's doing; can I have a diary too? -- he takes it and gives it to the shopkeeper. -- Oh, by the way, could you please write something in it with a pen, somewhere in the margins? Sometimes the ink dries up while it's just sitting in the store.
It's like a hint for her to write down what her problem is, if the diary won't be ruined by it, of course... If it will be ruined, then she'll give him her notebook.