Lenin: 156. LENIN AND KRUPSKAYA TO LENIN’S SISTER MARIA https://www.marxists.org/archive/lenin/works/1905/jun/00mu.htm
Marx-Engels Correspondenc: Nationalism, Internationalism and the Polish Question by Frederick Engels 1882 http://hiaw.org/defcon6/works/1882/letters/82_02_07.html
Comintern Archive: Marx and Engels on the trade-unions http://ciml.250x.com/archive/5classics/english/marx_engels_crisis/marx_engels_crisis.html