I'd start with Kropotkin's "The Conquest of Bread" followed by Gelderloos' "Anarchy Works." Kropotkin explains the theory behind why libertarian socialism / anarchist communism is a better, more fair way to structure society, while exhaustively addressing common objections. Gelderloos writes from a modern perspective while offering examples of non-hierarchal human organization throughout history. For those who (somehow) read these and remain convinced that the idea is utopian, I'd recommend Kropotkin's essay "Are We Good Enough," and his principal scientific work: "Mutual Aid: A Factor of Evolution" which offers a compelling counter-argument to the 'dog eat dog' conclusion drawn by many from Darwin's theory of evolution by positing that cooperation is sustained in humans and animals over time through natural selection.

For a quick and dirty intro to the basic idea of what classical anarchism is: "An Anarchist Program" - Errico Malatesta.

Anarchism is a line of political thought that goes back for well over a century, with many branches and differing opinions; like any group, libertarian socialists are not a monolith. Anarchism, including anarchist communism, is a response not only to capitalism, but to other branches of leftist thought: while communist revolutions were taking place around the world, anarchists were there alongside them, critiquing the practices that continue to be critiqued today concerning communist projects with their focus on challenging power and unnecessary hierarchy: who has power over who, why, and is it strictly necessary? As someone once said: "Freedom without equality is the jungle. Equality without freedom is prison. I want neither the jungle nor prison." That is what classical anarchist thought brings to the table: it examines how society can be structured while providing both freedom and equality in a way that neither capitalist thinking nor other leftist schools of thought adequately address.