As a Man Thinketh (1903) is the most widely read book James Allen wrote — a short meditation on the proposition that character is the sum of one's habitual thoughts, and that life is shaped by the inner pattern more than by outward circumstance. Where his contemporaries in the New Thought movement promised wealth and health, Allen wrote about character — the slower, harder, more durable claim.

The book has been continuously in print for over a century, has sold tens of millions of copies in every major language, and is the seed-text of the entire modern self-help tradition. Napoleon Hill, Earl Nightingale, Dale Carnegie, and the wave of twentieth-century motivational writers all worked downstream of Allen's small book.