This enameled tin cup has the slogan 為人民服務 Service to the People. The characters are traditional (complicated), which may mean that it was manufactured prior to the simplification of some of the characters on the cup. Today, the characters are written 为人民服务 in simplified form.
The sun behind three books of Mao’s quotes represents Mao, who is referred to as 红太阳, the Red Sun.