Hello, I'm trying to learn Japanese and would like some Japanese books on Samurai and/or pre-modernization Japanese warfare in general to help motivate my learning.
Hey, I replied to your PM, but I'll post the information here as well, in case anyone else is interested.
Most Japanese language instructional materials don't teach the grammar and vocabulary you need to read history books, unfortunately, which makes the jump from text books to history books rather painful. It's very specialized vocabulary, and often quite advanced grammar (the only place I've ever seen N1-level grammar commonly used). But when I was learning, I found 留学生のための日本史 very helpful. It is intended for language learners, so the grammar is more accessible, and the kanji has furigana to help you look up ones you don't know. Another nice thing is it does define some of the specialized vocabulary in English, but still uses the Japanese throughout the text. It also helps get you used to names and graphs in Japanese, which I thought was very helpful. This doesn't specifically cover pre-modern warfare, since it's a survey of the entire history, but it does touch on it. A little more than half of the book is devoted to pre-1854, which is actually a fairly generous proportion for most surveys.
As far as writers specifically writing about warfare, Karl Friday is the most prolific scholar in English, and I can't think of them off the top of my head, but he does have at least one article in Japanese as well. Tom Conlan has many good articles in English that just might be interesting for you, but to my knowledge hasn't published in Japanese. He does have a Japanese historical glossary in an appendix to State of War, which I've found very helpful in learning historical terms to be able to read Japanese scholars.
Keywords you might be able to use to find online reading (or books, if you have access to a university library - they are generally really expensive otherwise) are 中世、武士、武士団、戦国、悪党・海賊 (there's a really incredible book by Amino Yoshihiko on Akuto and Kaizoku, but again it's very difficult to read).
If you're interested specifically in Shinsengumi, there's a book called 新撰組顚末記 which was written by Nagakura Shinpachi. You can get it in paperback, the copy I linked to is translated into contemporary Japanese. It's not quite as difficult as reading historians' writings, I think.
If you don't have any luck here, you might be better off asking in /r/japan or /r/japanlife, or another Japanese-oriented subreddit. Most of the contributors here don't read Japanese, so you might have some difficulty getting an answer in AskHistorians.