The Khodarkovsky text you found is one of the best introductions to this subject. William Sunderland's Taming the Wild Field is another book that will compliment Russia's Steppe Frontier. The books on Russians' orientalism and the origins of the Russo-Japanese War by David Schimmelpenninck van der Oye cover the intellectual aspects of Russia's move to the east. Finally, I would be remisd if I did not mention Yuri Slezkine's Arctic Mirrors, which examines the cultural underpinnings of the relationship between Russia and the various peoples inhabiting Siberia.