Resources has a lot of useful materials translated into English on the Cultural Revolution

Compilation of Cultural Revolution Sources

Works of Mao Zedong:
Selected Works of Mao Zedong
Selected Quotations of Chairman Mao (Little Red Book)

CCP sources:

Peking Review
Circular of the Central Committee of the Communist Party of China on the Great Proletarian Cultural Revolution

Jiang Qing
Lin Biao Reference Archive
Yao Wenyuan Internet Archive

Chinese Language Sources

Red Flag 红旗 1958-1976

People’s Daily 人民日报 1946-1978