Togo's diary in English for 1874 is transcribed (of course in English) and published unfortunately with the annotation in Japanese as the following article in the bulletin journal of a Japanese university: MIYANAGA, Takashi. 'On Heihachiro Togo's Diaries in English'. Shakai-shirin: Hosei journal of sociology and social sciences 48-2 (2001): 47-128 (in a reversed order), at pp. 47-105.

According to the introduction, the original notebooks are now found in Togo shrine.

I can only find one possibly published source on this. That is Admiral Togo: a memoir, compiled by Koya Nakamura, under the supervision of Tetsujiro Inouye (Tokyo : Togo Gensui Pub. Society, 1935).

Other possible (English) sources which might reference the diary:

Clements, J. (2010). Admiral Tōgō: Nelson of the East.

Busch, N. F. (1969). The Emperor's sword: Japan vs. Russia in the Battle of Tsushima. New York: Funk & Wagnalls.

Blond, G. (1958). L'amiral Togo: Samouraï de la mer. Montreal: A. Fayard.

Falk, E. A. (1936). Togo and the rise of Japanese sea power. New York: Longmans, Green and Co.

Ogasawara, N. (1934). Life of Admiral Togo. Tokyo: The Seito shorin Press.

Blond, G. (1961). Admiral Togo. London: Jarrolds.