To read more about the Forbidden City, here is a list of useful books.

For Children:

Chiu Kwong-chiu, In the Forbidden City. China Institute in America: 2014.
Adeline Mah, China: Land of Dragons and Emperors. Delacorte: 2009
Brian Tse, This is the Greatest Place! The Forbidden City and the World of Small Animals. China Institute in America: 2014.

For Adults:

Geremie Barme, The Forbidden City. Harvard: 2012
Gilles Beguin and Dominique Morel, The Forbidden City: Center of Imperial China. Abrams: 1997
Wing-Tsit Chan, A Source Book in Chinese Philosophy. Princeton University Press: 1967
Lin Yutang, Imperial Peking: Six Centuries of China. Elek Books: 1961
Roderick MacFarquhar, The Forbidden City: China’s Ancient Capital. Reader’s Digest Association: 1972
Mary C. Wright, The Last Stand of Chinese Conservatism: The T’ung-Chih Restoration, 1862-1874. Atheneum: 1967
Wu Hung, Remaking Beijing: Tiananmen Square and the Creation of a Political Space. University of Chicago: 2005
Madeleine Yue Dong, Republican Beijing: The City and its Histories. University of California: 2003