The Sengoku period marks the beginning of widespread formalised martial schooling in Japan, modelled after the transmission and licensing systems inherent to fine arts such as theatre, music, and the like (although systems of weapons training and actual schools can be identified from earlier periods, they were mainly endeavors like archery and were rather different from the later schools, which are commonly termed koryu budo/bujutsu/bugei today). I believe Legacies of the Sword by Dr. Karl Friday gets into some detail on this.

These schools did write things down in scrolls and books, particularly during the peaceable Edo period when there was ironically an explosion of martial arts. However much like other crafts, koryu were beholden to varying levels of intrigue, and to this end proprietary secrets such as techniques would be mostly conveyed in person and hands-on. What does survive written down is often unintelligible without further background in the teachings of the school, and there are more than a few instances where they survive but as relics to heirs who have lost the tradition. Koryu Bujutsu and other essay collections edited by Meik and Diane Skoss offer good perspective here.

I'll add that one interesting exception to the status quo would be Chinese “boxing manuals” that circulated into Japan throughout the Edo period. Ellis Amdur elaborates on the topic in a section of his Hidden in Plain Sight.