For the purpose of investigating the books, accounts, documents, or papers of any person, association or corporation, liable by law to make report to the Department of Revenue, or any county officer acting as agent for the Commonwealth, for the purpose of taxation, the department may employ, from time to time, one or more expert accountants, who shall have power to inspect the same and report thereon, but any such investigation shall be limited to such of the books as bear upon the subject matter of the tax under investigation.
72 P.S. § 212