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 that person, to attend before it, or any of its members, and bring for inspection any books, accounts, or papers, of that person in that person’s possession and under that person’s control, and to testify under oath touching any matter relating to the organization or business of that person.