Biography of Li Hongzhang by Liang Qichao, Beijing: Baihua Literature and Art Publishing House, May 2000.

The most practical:

Yuan, Biography of Li Hongzhang. Beijing: People's Publishing House, 2004.5.

What you want:

Wang Peiyin. The Truth of Li Hongzhang's Missions to Europe and America. Beijing: Chinese Publishing House, 2006.

The most detailed:

Complete works of Zhou Dianlong, Sun Suyun and Li Hongzhang. Changchun: Times Literature and Art Publishing House, 1998.

Others:

Zhang: Looking at History with Different Eyes: Li Hongzhang in the Eyes of Foreigners. Changchun: Jilin Photography Publishing House, 2003.

Hehe, Xiao Sheng is a history student. I have read some books about Li and written several papers at school. The above are the reference materials I use. I hope it helps ~!