What are the business ledgers used for?
Once you officially join the Thieves' Guild, you'll be able to pick up additional jobs from Devin Mallory and Vex: they'll assign you one of seven types of random quests for you to do.
One of the quests, The Numbers Job, will task you with forging/modifying a random business ledger throughout the world. That's what you need them for.
Outside of that, they're not used.