Reddit mentions of Economics: A New Introduction

Socialism/Communism

A People's History of the World

Main Currents of Marxism

The Socialist System

The Age of... (1, 2, 3, 4)

Marx for our Times

Essential Works of Socialism

Soviet Century

Self-Governing Socialism (Vols 1-2)

The Meaning of Marxism

The "S" Word (not that good in my opinion)

Of the People, by the People

Why Not Socialism

Socialism Betrayed

Democracy at Work

Imagine: Living in a Socialist USA (again didn't like it very much)

The Socialist Party of America (absolute must read)

The American Socialist Movement

Socialism: Past and Future (very good book)

It Didn't Happen Here

Eugene V. Debs

The Enigma of Capital

Seventeen Contradictions and the End of Capitalism

A Companion to Marx's Capital (great book)

After Capitalism: Economic Democracy in Action

Capitalism

The Conservative Nanny State

The United States Since 1980

The End of Loser Liberalism

Capitalism and it's Economics (must read)

Economics: A New Introduction (must read)

U.S. Capitalist Development Since 1776 (must read)

Kicking Away the Ladder

23 Things They Don't Tell You About Capitalism

Traders, Guns and Money

Corporation Nation

Debunking Economics

How Rich Countries Got Rich

Super Imperialism

The Bubble and Beyond

Finance Capitalism and it's Discontents

Trade, Development and Foreign Debt

America's Protectionist Takeoff

How the Economy was Lost

Labor and Monopoly Capital

We Are Better Than This

Ancap/Libertarian

Spontaneous Order (disagree with it but found it interesting)

Man, State and Economy

The Machinery of Freedom

Currently Reading

This is the Zodiac Speaking (highly recommend)

I don't know if it's essential reading, but Thomas Meyer's The Theory of Social Democracy is an up to date theoretical exposition.

When it comes to economic thought in particular, my favourite is Hugh Stretton's Economics: A New Introduction, which favours an institutionalist, post-Keynesian approach to economics.