APA (7th ed.) Citation

Gunn, J. E. (1977). This fortress world. Sphere Books.

Chicago Style (17th ed.) Citation

Gunn, James E. This Fortress World. London: Sphere Books, 1977.

MLA (9th ed.) Citation

Gunn, James E. This Fortress World. Sphere Books, 1977.

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