A more thorough explanation:

A letter-book is a book used by merchants to store their correspondence. It is a type of ledger that contains copies of letters sent and received by the merchant.

• A merchant may keep a letter-book to keep track of orders, invoices, and other important business communications.
• John's letter-book contained copies of all the letters he sent to his suppliers and customers.

These examples illustrate how a letter-book is used to keep track of important business communications. By keeping copies of all letters sent and received, merchants can refer back to them later if needed.