London's most famous book was certainly The Call of the Wild (1903), a story of Alaskan adventures told through the eyes of a dog named Buck. London originally planned it to be a short story of about 4,000 words but it quickly became a novel. Full of both pathos and violence, the book was a gritty and dark look at the nature of both man and beast:
He was beaten (he knew that), but he was not broken. He saw, once for all, that he stood no chance against a man with a club. He had learned the lesson, and in all his afterlife he never forgot it. That club was a revelation. It was his introduction to the reign of primitive law... The facts of life took on a fiercer aspect, and while he faced that aspect uncowed, he faced it with all the latent cunning of his nature aroused. (From Chapter 1)
The work was first published by the Saturday Evening Post, which paid the cash-strapped London $750. When printed in book form, Macmillan publishers worried about taking a risk on the relatively unknown writer. Instead of offering royalties, they paid London $2,000 outright. When it became a huge success, London made no additional money.
But when still a teenager, London already had cash problems — that and a need for adventure propelled him to sea. On his 17th birthday, January 12, 1893, he acquired a space on board the schooner Sophie Sutherland, bound for Japan to hunt seals. Already known locally as a bit of a ne'er-do-well with a drinking problem, London set sail about a week later. This experience inspired the next novel he published in his own name after The Call of the Wild. He named this one The Sea-Wolf, published in 1904.
*I must acknowledge a debt to James L. Haley, who provided some of the information in this post in his 2010 book Wolf: The Lives of Jack London.