"Three Books" are documents used in the process of marriage, which can be said to be an effective written record to ensure marriage in ancient times. It means:

Uncle San

Letter of appointment: namely letter of appointment. When Naji (a man and a woman are engaged), the man's family gives the book to the woman's family. Gift book: a document used in the ceremony, which lists the items and quantities of the ceremony. Wedding book: that is, the document to marry the bride. It is the certificate that the man gave to the woman when he came in to greet the bride.