Book of Five Rings by Musashi Miyamoto
There are a handful of English translations of this book, some better than others. It will give you something to think about, but it will be easier to understand the more advanced you become. Don’t rush to finish the book, but rather consult it from time to time and see what it inspires you to perceive.
The Hagakure by Yamamoto Tsunetomo
There are many translations of this book as well, but it goes into the pragmatic realities of Samurai life. You may not reach satori reading this, but you can gain some interesting insight into the path of the world’s most famous warriors.
Bushidō
There are many books on this subject, some of which are better than others, but none of which really admit that the Bushidō code was an academic creation later in the history of the Edo period to try to stimulate the Samurai who were no longer fighting wars, and could become restless. Books on this subject by Western historians often attribute the practice of Bushidō to the entire Samurai era when it was really more a thing in the late Edo period. There is a lot of influence from Confucianism, and the development of Bushidō can be seen to a certain degree as an attempt to tame the Samurai. There are interesting things to be found, but everything must be taken with a grain of salt.
Zen
There are many books on Zen, even some that are particularly focused on Zen in martial arts, but Zen is a separate path from the warrior’s path. Many Samurai practiced Zen as a way to enrich their spirituality, but it is not necessary. It is true that Zen had a big influence on all martial arts in Japan, and some say that just meditation practice counts as Zen. I consider meditation a technique for exercising the mind, and Zen to be a form of Buddhism. Read about Zen if you are curious, and you may get some useful insights, but you don’t need to become a Zen monk.