APA (7th ed.) Citation

Tolkien, J., Carpenter, H., & Tolkien, C. (1990). Letters of J.R.R. Tolkien. George Allen & Unwin.

Chicago Style (17th ed.) Citation

Tolkien, J.R.R, Humphrey Carpenter, and Christopher Tolkien. Letters of J.R.R. Tolkien. London: George Allen & Unwin, 1990.

MLA (9th ed.) Citation

Tolkien, J.R.R, et al. Letters of J.R.R. Tolkien. George Allen & Unwin, 1990.