In UK, 'printed matter' refers to what is contained inside the envelope: documents, newspapers, magazines, letters etc. Basically, it is things made of paper! It is used because it can affect the cost of posting the envelope - some mail services charge different amounts, depending on what kind of things are in the envelope.

Books, letters, documents, etc. are all printed matter. It just means anything that has been printed. About the grammar: "printed matter" is a noncount noun (like rice, water, money, milk, information, etc.) so you CAN'T say "printed matters." You can just say, for example, "This package contains printed matter," or, "Is there any printed matter in this package?"