"Bullshit Jobs". Also the Revolutions podcast. That one is especially good cause it sneaks on you. You start listening to it as a lib and before you know it you realise how it's the same struggle and the same types of people resisting change and making things good for everyone and boom...you're radicalised approximately by the time you're done with the Haitian revolution

People’s History of the United States

Theory is just likely to bore them or piss them off

Blackshirts and Reds. It covers some very important history, illustrates how capitalism decays into fascism and is destroying us and the planet (and has done so in the past), introduces some baby steps into Marx at the end of the book, and debunks many anticommunist talking points. Also it's more modern writing so the lib can actually understand it without a dictionary in the other hand, is quite funny at points, and hits them with the emotional punches they need. I cry every time I read it.

It is not technically theory, but the two things that radicalized me the most were Howard Zinn's A People's History of the United States and Mike Duncan's Revolutions podcast. I found that with a foundation of history told through a materialist lens, most of the more theoretical texts simply clicked. The problem you'll face most of the time is that Americans (including liberals) simply don't know enough history, and where they lack in historical knowledge, context, and hindsight, they improvise with idealism. The only way you can get them thinking in a materialist lens is if you stuff their brains with history.

Think about it. Liberals are always obnoxiously telling people how things are supposed to work. They are the self-appointed experts on how things are supposed to work. We have to break that illusion and show them how things have and do work.

Against Empire. The Shock Doctrine (to a lesser degree). Anything that describes the enormous violence which is required to uphold the status quo.

Theory isn't radicalizing.

History and lived experience are.

Since you can't just hand someone a book for lived experience, I suggest Open Veins of Latin America or People's History of the United States to start.