[Luckily for Will, Emma has an answer ready at the get-go. She's prepared to explain these to anyone James hasn't already reached.]

If anyone picks them up but the person they belong to, they're rendered useless. They're warded, so while we can't send private messages through them -- yet, anyway -- if anyone else tries to grab hold of your notebook and take a peek, the spell will react and show them nothing but blank pages. Maybe sling an insult or two.

[James had to add a personal touch, after all.]