I'm going to take a different tack than some others are here. Instead of giving you classics, I'm going to recommend mostly things from the "other" direction, mostly critiques of our current world written by leftists, or histories of leftist ideas. These will probably be more familiar to you, but will introduce you to new ideas as you go, which you can dig into as you get deeper.
I don't know what kind of interests you have, but here's a few:
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Utopia of Rules: A collection of 3-4 essays about bureaucracy. Everyone hates bureaucracy, but somehow the right has monopolized hating bureaucracy in American politics. A nice place to start that will resonate with most people.
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The People's Republic of Walmart: A relatively concise history of socialist planning (as opposed to market economies), which includes the simple yet no less profound insight that megafirms like walmart already do major economic planning on the scale of countries, and it works. A nice book, though fair warning, the prose can be a bit tedious sometimes. Not too technical, but technical enough that it could pique your interest if you are inclined towards those kinds of things as I am.
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Cybernetic Revolutionaries: If you like history, this one might be for you. It's the story of Allende's government's Cybersyn project. Eden Medina did a wonderful job interweaving the concepts of cybernetics, politics, and the history in a really important way, and one that contradicts the trend in our world to separate "politics" from everything.
Happy to recommend something else if none of these are to your liking.