...is Dr. Benjamin Spock:
In 1946, Spock published his book The Common Sense Book of Baby and Child Care, which became a bestseller. Its message to mothers is that "you know more than you think you do." By 1998 it had sold more than 50 million copies. It has been translated into 39 languages. Later he wrote three more books about parenting.
According to The New York Times, Baby and Child Care was, throughout its first 52 years, the second-best-selling book, next to the Bible. According to other sources, it was among best-sellers, albeit not second-best-selling.
I came from a Spockian family. My mother swore by the book, which is still a refreshing, reassuring, jargon-free guide, despite the fact that it was written under the influence of psychoanalysis. Spock had the practical intelligence that teaches you not to be impressed by jargon, but also to realize that there may be useful ideas concealed beneath it. Our family talked about "Dr. Spock" so much that I always called Mr. Spock from Star Trek Dr. Spock, and thought they were somehow related, perhaps even the same person.
Spock also came out swinging against the Vietnam War, denouncing it every chance he got and getting himself arrested during protests. I'd be willing to wager that this is one of the rare cases in modern history in which a writer has actually influenced mass opinion on an issue of actual importance. Dr. Spock found it objectionable that an entire generation of young men he had helped raise were being sent across the earth to fight, kill, and die in some steaming jungle in a pointless, savage war.
So writers, if you really want to influence the world, all you have to do is write one of the best-selling books in history addressing a topic of concern to every living human!