The ancient ledger is a relic that may be received from breaking open barronite deposits. A relic from the Second Age, it belonged to the inhabitants of Lassar, who were destroyed by Saradominist armies after the events of the Betrayal of Zaros, which included the Imcando dwarves.
The ledger can be handed to Curator Haig Halen at the Varrock Museum for 1 Kudos and 200 coins. Players will receive 100 coins for additional ancient ledgers. When given to the curator for the first time, he will comment "I wonder what secrets this Ancient ledger holds. It's a unique find!"
For an exhaustive list of all known sources for this item, see here