There are a lot of books about the Shinsengumi. I have referred to many of them in my research and writing thus far, including in Shinsengumi: The Shogun’s Last Samurai Corps. I am relying on them as I research and write my next book. Some of the best ones from a purely historical viewpoint are listed below:

Hirao Michio. 定本　新選組史録　(Teihon Shinsengumi Shiroku)

Kojima Masataka. 新選組余談　(Shinsengumi Yodan)

Kojima Masataka. 武術天然理心流　(Bujutsu Tennenrishinryu)

Matsuura Rei 新選組 (Shinsengumi)