I’ll let somebody else hit you with the theory and whatnot, but in my own journey pushing myself left I started with Zinn’s A People’s History and Loewen’s Lies My Teacher Told Me to unprogram all the crap they taught you in school.

Then while not necessarily leftist, to understand modern politics, I’d recommend Listen, Liberal by Thomas Frank and Dark Money by Jane Mayer noting that while Mayer pretty much gives the Dems a free pass, Frank points out that they were benefiting from the Koch brother’s antics right up until about 2014. MacLean’s Democracy in Chains is a good background on neoliberal’s origins and how they linked with the Koch brothers.

You can also give The New Jim Crow by Michelle Alexander a read, but honestly you could watch 13th and get the same understanding of the modern justice system’s racism.

Again, this all not necessarily Leftist, but it’s good for ripping the band aid off the lib shit we were taught.

a classic first introductory text is zinn’s people’s history

I’ll second Zinn’s A People’s History, but I’ll also add Killing Hope if you’d like to know about the history of the imperialist stuff the US did in the 20th century like the overthrown of Allende and support for the Contras.

October by China Melville tells a great story like account of the October revolution

Revolutions podcast gives a pretty good account of some important events too.