APA (7th ed.) Citation

Nicolson, H. (1939). Why Britain is at war. Penguin Books.

Chicago Style (17th ed.) Citation

Nicolson, Harold. Why Britain Is at War. Harmondsworth: Penguin Books, 1939.

MLA (9th ed.) Citation

Nicolson, Harold. Why Britain Is at War. Penguin Books, 1939.

Warning: These citations may not always be 100% accurate.