Socialism: Utopian and Scientific by Friedrich Engels. Short, yet clearly elaborates on the shift from earlier utopian views of socialism of figures like Robert Owen to the scientific socialism of Marx and Engels.

Honorable mentions go to Capital, Volume I by Karl Marx, Imperialism, the Current Highest Stage of Capitalism by Lenin, The Wretched of the Earth by Frantz Fanon, and Neocolonialism, the Last Stage of Imperialism by Kwame Nkrumah. This series of books expands on capitalism during its beginning, intermediate, and imperialist phases, and imperialism itself in its beginning, intermediate, and final phases as they relate to colonialism and neocolonialism.