Someone else asked exactly the same question a few months ago — here are some texts I've used in that past that I recommended to them:

• W.G. Beasley, Japanese Imperialism, 1894-1945 (1987)
• Myers & Peattie, eds., The Japanese Colonial Empire, 1895-1945 (1984)

From the Russian angle, see:

• Wolff & Steinberg, The Russo-Japanese War in Global Perspective: World War Zero (2007)
• David Schimmelpenninck van der Oye, Toward the Rising Sun: Russian Ideologies of Empire and the Path to War with Japan (2006)

As a general source on the 1905 war that's useful if a little aged now, have a look at:

• Dennis & Peggy Warner, The Tide at Sunrise: A History of the Russo-Japanese War, 1904-1905 (1974)

Hope that helps!

In addition to k1990's recommendations, I would also suggest the relevant chapters in David Evans and Mark Peattie's Kaigun: Strategy, Tactics, and Technology in the Imperial Japanese Navy