Tax books. Records kept by a firm's management that follow IRS rules. The books follow Financial Accounting Standards Board rules.

The Internal Revenue Service (IRS) Data Book is published annually by the IRS and contains statistical tables and organizational information on a fiscal year basis. The report provides data on collecting the revenue, issuing refunds, enforcing the law, assisting the taxpayer, and the budget and workforce.

M-1 adjustments: reconciliation of book and taxable income (income and deductions.) Differences exist because of the difference in GAAP and tax law.

Remember that all income, no matter the amount, is taxable unless the law says otherwise – even if you don't get a Form 1099-K. If you get money from someone as a gift, reimbursement or repayment of other personal expenses, that money is not taxable.