Write down what interests or bores you. Speculate about why. If you get stuck in the reading, think and write about where you got stuck. Contemplate why that particular place was difficult and how you might break through the block. Record and explore your confusion.
Confusion is important because it's the first stage in understanding.
When the going gets difficult, and you don't understand the reading, slow down and reread sections. Break long assignments into segments. Read 10 pages, then do something else. Later, read the next 10 pages and so on. Read prefaces and summaries to learn important details about the book.