In accounting, "the books" is everyday shorthand for a business's accounting records, and above all its general ledger, the master set of accounts from which the financial statements are prepared. The phrase dates from the time when ledgers and journals were literally bound volumes.
What the books include
The books normally cover every record in the bookkeeping system. That includes the journal, where transactions are first recorded in date order; the general ledger, where they are sorted by account; and supporting records such as each subsidiary ledger for individual customers and suppliers. Today almost all of these live in accounting software, but the terminology has survived.
Several common phrases come from the term. To "keep the books" is to record a business's transactions. To "close the books" is to complete the period-end process, after which entries for that period are locked. Figures "per books" are the company's own recorded amounts, as opposed to amounts per the bank or per a third party; for instance, the balance per books is compared with the balance per bank in a reconciliation. "Cooking the books" is slang for deliberately falsifying the records.
A business's books are also what auditors examine and what tax authorities may request. Keeping them complete and up to date, with a balanced trial balance at each month end, is the foundation of reliable reporting.