APA (7th ed.) Citation

Tolkien, J. R. R., Carpenter, H., & Tolkien, C. (1981). Letters of J.R.R. Tolkien: A selection. Allen & Unwin.

Chicago Style (17th ed.) Citation

Tolkien, J. R. R., Humphrey Carpenter, and Christopher Tolkien. Letters of J.R.R. Tolkien: A Selection. London ; Boston: Allen & Unwin, 1981.

MLA (8th ed.) Citation

Tolkien, J. R. R., et al. Letters of J.R.R. Tolkien: A Selection. Allen & Unwin, 1981.