Comment by Morromist

Its by Augustus F. Lindley and called Ti-ping tien-kwoh; the history of the Ti-ping revolution Volume I and II. And its actually 955 pages. Its an account written by one of the generals fighting for the rebels, who was also a British guy. Its part history part memoir. Kind of a weird book.

I suspect those other books people mentioned are probably better if you want a good understanding of the war that's not one sided and written in 1866. But its one of the very few contemporally written first-person works available.

Edit: you can find it on Project Gutenburg for free if you want to take a look at it. https://www.gutenberg.org/files/39180/39180-h/39180-h.htm