The banking book of a bank is the portfolio of assets, primarily loans, a bank expects to hold until maturity when the loan is repaid fully; typically refers to the loans the bank underwrites.

Definition An accounting book that includes all securities that are not actively traded by the institution, that are meant to be held until they mature. These securities are accounted for in a differe [..]

Bank activities are classified into two groups - the banking book and the trading book. The banking book includes the loans, some bonds, and deposits. Its key accounting characteristic is that assets are recorded at historical cost.

This is the portfolio where assets that are purchased for the purpose of long term investment are held. It is normally accounted for on an accrual basis, (see trading book).