"Author" of Hagakure, which is a book of selected transcriptions of conversations between himself and Tashiro Tsuramoto, who acted as scribe.
Yamamoto was of the samurai class, and acted as retainer to Nabeshima Mitsushige, third daimyo of the prefecture of Saga. Nabeshima died on May 16, 1700. Traditionally, samurai would follow their masters into death by committing seppuku (the practice known as junshi), but Yamamoto was prevented from doing this by his master's edict which ended the junshi custom. The Tokugawa Shogunate, then rulers of Japan, had also outlawed junshi (although not seppuku).
As such, Yamamoto retired at the age of 42 and became a Buddhist monk. He lived at Kurotsuchibaru (on the island of Kyushu) in near seclusion for ten years.
Tashiro began visiting Yamamoto in 1710, and they met regularly for seven years. Tashiro published Hagakure, his lessons from Yamamoto, in 1716.
Yamamoto's strict adherence to the samurai way was somewhat out of date, even at the time. His moral code is anachronistic by approximately (according to William Scott Wilson, translator of Hagakure) one hundred years.
Information gleaned from the introduction to Hagakure.
Thanks to Shro0m for the kanji.