Written in 1880 as a popular pamphlet, Engels' Socialism: Utopian and Scientific is much more than a simple condemnation of the evils of capitalism. It is, as John Rees says in his introduction, a startling book, combining a clear account of the development of capitalism with a survey of the battle for socialist ideas in the new working class and discussion of questions of method and philosophy.
Very few authors manage to accessible writing with deep insight into how society works. Fewer still manage to combine both with brevity. Frederick Engels, perhaps even more than Karl Marx, was one who did. One of the best concise accounts of capitalist development, Socialism: Utopian and Scientific is an excellent introduction to Marx's revolutionary socialism.