Hagakure: Book of the Samurai
The book preaches bushido, the warrior code of the samurai. Hagakure asserts that bushido is really the "Way of Dying" or living as though one was already dead, and that a samurai retainer must be willing to die at any moment in order to be true to his lord. However true to its title, the wisdom from this book can be applied in everyday life to anyone (just read between the lines), and even Tsunetomo says that a warrior does not necessarily have to know martial arts.
Hagakure was not widely known during the decades following Tsunetomo's death. However, it received wider circulation at the start of the 20th century, and by the 1930s had become one of the most famous representatives of bushido thought in Japan. After World War II, it has acquired a very bad name in Japan as one of the influences that lead the nation to militarism and ruler-worship, and ultimately to defeat. However Hagakure remains popular among many non-Japanese who are interested in samurai culture; It is also frequently referred to as The Book of the Samurai and was featured prominently in the 1999 Jim Jarmusch film Ghost Dog - The Way of the Samurai.
After his master died, Tsunetomo himself was forbidden to commit seppuku, a retainer's ritual suicide by the edict of the Tokugawa Shogunate. Hagakure was written partially in an effort to outline the role of the samurai in a more peacetime society. In fact, Tsunetomo himself is thought to have never been involved in a battle or duel.