I'd personally use them for snacks. But you do you.
From our friends at craighill:
Some secrets are best kept hiding in plain sight. These three seemingly technical volumes blend in admirably among the rest of your library, but they hold a secret. Inside, you won’t find meditations on texture, tips for laser safety, or tales of a machinist’s woe. Instead, an empty cavity awaits, ready to take hold of your secrets and keep them safe — relatively speaking.
