For me, it depends.

I don't fully understand anything till I've read it multiple times. Not necessarily the whole thing. It helps to take breaks between reads. I can read a text today, tomorrow, next week, and next month and arrive at completely different interpretations/conclusions every time.

So if I really need to understand a text, I'll read it and re-read it, with different length breaks in between.

Most of the time I don't need to bother with such depth. For example I might read 'Better Fewer, but Better' and get a lot from it, making sure the main conclusions are clear. Then if I read it a year later, a lot of the details will jump out at me that I hadn't noticed much before. Like the names of the journals that Lenin and his comrades/enemies/frenemies are writing in, the dates of different movements (how slow things were and then how fast came the revolution), the nuances in the arguments of the different movements/groups. While this is all interesting to know, it's not always essential. So I have to make a choice: read widely or read closely; do I read the same text twice or two texts, for example?

Then there are texts that I need to do something with. Maybe I'm writing something. If so, I'll make notes. I use four methods depending on what I need.

1. Annotating. Reading a physical text, I usually underline key phrases/words (or highlight if digital), draw a line down the margin instead of underlining a whole paragraph, use asterisks for key quotes, question marks (for things I don't understand or which don't add up), ticks (for parts that seem incontrovertible), exclamation marks (for parts that are factually shocking or shockingly incorrect), ellipses (where I think a point needs to be expanded further because it's interesting, poorly evidenced, or would lead me down an interesting path). Using two or three symbols for emphasis.
2. Marginal notes. I might write a question. Or write a word or two next to each paragraph summarising it's main 'move' so that I can recap the whole argument by reading a string of these moves in the margin. This helps to keep track of complex arguments. Or I might use one word to describe each premise in a section/argument (I'll write these at the start of the section so I have my summary of the argument next to the heading).
3. Copying select quotes into a document or a physical notebook, adding some analysis, criticism, synthesis, and evaluation. I'll focus on the most interesting parts and get to know a few sections really well.
4. If I really need to know a text, I'll copy the text (or a section/para/chapter) into a word processor, then go through and reword each paragraph with my own summary, keeping the 'quoteable' words/phrases/paragraphs as quotes. In this way I'll create a summary of the whole text, to which I'll add criticism, synthesis, and evaluation.

Honestly, though, the most effective method for me is just to read. I prefer epubs so that I can read on a smaller screen (e.g. a Kindle). Then I'll read while I'm waiting for the kettle to boil or the tea to brew or while I'm otherwise waiting (for buses, meetings, colleagues, any down time, really). I wouldn't be able to go through the intense process described above for many texts or I wouldn't get through enough literature. (The problem with PDFs is that I can only read them on a computer, which limits when I can read them.)

I'm not a huge fan of audiobooks but I do use them when doing chores, walking, etc. I prefer podcasts or lectures for this, though. There's something about the cadence and rhythm of an audiobook—it just doesn't seem to go in the same way as reading the same text myself.

Maybe it also depends on the author? Zac Cope or another 'Western academic Marxist'? If I don't follow the annotations and marginal notes methods, above, as a minimum, I won't understand a word.

I think 'just reading' works best for me because I'm like a sponge when the stakes are low and I'm just chilling. Almost as soon as I make it 'work', when I've decided that I need to know the text inside out, it all becomes so intense that I go sloooowly and it can get quite stressful, re-reading every line to make sure I understand it perfectly (an impossible standard). I learn a lot quicker and take in a lot more if I keep it light and keep the pressure off; I can always go back in more depth later if I need to.

Hopefully this waffle is helpful! ❔❓

What are you reading theory for, DankZedong?