I'd recommend 'Ideology and Ideological State Apparatus' as Althusser's foundational text. His essay 'On the Materialist Dialectic' is the one where he talks about what I brought up. It can be tricky to understand if you haven't read much dialectic theory before Althusser, but he basically argues that there's a plurality of economic classes and activities, each with some degree of autonomy, but all of them depend on one another to a degree that they shape the boundaries of the other.

I will point out that in very clear terms that Althusser's own battle with mental illness shaped much of his philosophical work. He was very interested in structure and how various people were slotted into formations completely outside of themselves. He had a lifelong battle with schizophrenia that had him institutionalized at various points and in one very severe episode he accidentally killed his own wife.

He didn't believe in free will, is what I mean.