Cal. Gov. Code § 15617

The board may require anybody having knowledge of the business of any person who is or whose property is subject to assessment by it, or having the custody of the books, accounts, and papers of such person, to attend before it, or any of its members, and bring with him for inspection any books, accounts, or papers, of such person in his possession and under his control, and to testify under oath touching any matter relating to the organization or business of such person.