The Drowned Cipher

Venn's Curiosities has been shuttered since old Maret Venn vanished six weeks ago. You are her niece, let in by the landlord to take stock before the lease expires. The shop smells of cedar and damp. A single lamp burns low on the counter, and outside the backstreet puddles lie perfectly still, as though they are listening. You have found something in the ledger — a line written not in ink but in wax, invisible until you tilted the page into the light.