Lenin versus Trotsky and his followers https://marxistleninists.org/Trotskyism/Lenin/The%20struggle%20of%20Lenin%20and%20CPSU%20against%20Trotskyism.htm
Letters: Marx-Engels Correspondence 1869 http://hiaw.org/defcon6/works/1869/letters/69_08_18-abs.html
Letter To The Presidium Of The Conference Of Proletarian Cultural And Educational Organisations https://www.marxists.org/archive/lenin/works/1918/sep/17.htm