The secretary, or an assistant whom he may designate, may, either of his own motion or upon the verified complaint of any interested person, investigate the record of any person applying for or holding a license, or his agents.
For the purpose of such investigation the secretary or assistant may examine the ledgers, books of account, memoranda or other documents of any such person and may take testimony thereon under oath, but information relating to the general business of any such person disclosed by the investigation and not related to the immediate purpose thereof shall be deemed of a confidential nature by the secretary or assistant.