Artem the Gambler approached you after seeing that you had agreed to recover Elder Erza's ledger. He proposed that you bring it to him rather than to Erza.
Elder Erza's ledger lies within Erza's Library in The Last Archives.
You have found Erza's Ledger now you must return to The Council Chambers and decide whether you want to give it to Erza or Artem.
You brought the ledger to Artem rather than Erza and thus earned his gratitude.