Cal. Civ. Proc. Code § 337a

The term “book account” means a detailed statement which constitutes the principal record of one or more transactions between a debtor and a creditor arising out of a contract or some fiduciary relation, and shows the debits and credits in connection therewith, and against whom and in favor of whom entries are made, is entered in the regular course of business as conducted by such creditor or fiduciary, and is kept in a reasonably permanent form and manner and is (1) in a bound book, or (2) on a sheet or sheets fastened in a book or to backing but detachable therefrom, or (3) on a card or cards of a permanent character, or is kept in any other reasonably permanent form and manner.