APA (7th ed.) Citation

Ayer, A. J. (1956). The problem of knowledge. Penguin Books.

Chicago Style (17th ed.) Citation

Ayer, A. J. The Problem of Knowledge. Harmondsworth, Middlesex: Penguin Books, 1956.

MLA (9th ed.) Citation

Ayer, A. J. The Problem of Knowledge. Penguin Books, 1956.