It is easy to see the countries of Eastern and Central Europe as one foggy mass, because Russia's presence is so overwhelming and huge. I remember Milan Kundera's lament that in Western history books, Jan Hus has to share his space with Ivan the Terrible, even though they belong to very different cultural traditions and wouldn't understand each other at all.
This "East as a whole" concept is probably deeply rooted in the 19th century Pan-Slavic movement, whose leader was indeed Russia. The main idea was to unite all Slavic people under the rule of the tsar and under Orthodox Christianity, and to drive the Ottomans out of the Balkans. According to this idea, Slavs who were culturally different were treated as traitors (those who were Catholic or Protestant, for example), and various efforts were made to suppress and subjugate them. Also, the 19th century Russia was incredibly ethnically diverse – there were not only Slavs of various cultures and creeds, but also Jews who belonged to various religious movements (especially in Polish-Lithuanian Commonwealth), Tartars, Balts, settlers from Low Countries, settlers from various German states, Mongolic people like Buryats, Oirats, Kalmyks etc. etc., and Slavophiles tended to downplay that diversity.
If you don't want to go back further than the 19th century, I would look into the history of Pan-Slavism in Russia and Slavophile nationalist movements in general, and so-called "westernism" in Russia. I think it is important to know Orthodox Christianity and thought as well (it is NOT the same as Catholicism), and the history of Jewish communities in Eastern Europe.
There are tons and tons of books about this particular corner of the world, but I'm afraid I don't have a good grasp of what is available in English. One name I can recommend is Norman Davies for Polish history – "God's Playground", and "White Eagle, Red Star: The Polish-Soviet War, 1919 – 1920" – the last one is a bit old but good.
