Gekiken Jintsuu Roku
The Martial Arts of Japanese Fencing
Written about 1768, by Kashibuchi Arinori, this is an antique Japanese book on swordsmanship, as well as various training tools. This includes body armour for freeplay, which predates the design that we see in modern Kendo, or even pre-WWII Kendo. In fact, the words "Gekiken" 撃劔 is the historical term for sword sparring.
Interestingly, although this is supposed to be a Japanese publication, but more than 75% of it was written in classical Chinese. Which means that perhaps a Chinese reader would be able to understand this manual better than a Japanese.
This is a physical printed book, which contains the scans from this ancient manual. Purchase at: https://www.lulu.com/en/us/shop/jack-chen/gekiken-jintsuu-roku-the-martial-arts-of-japanese-fencing/paperback/product-1rvk84ke.html