you could take notes outside the book, just on a piece of paper, writing down key ideas or chapter summaries. It's fine to read around the subject too - look to see if you can find a companion guide, a simple-English explanation, maybe a lecture on a similar subject if switching up the format will be a help to you?

When I'm reading something theoretical I'll write one to two sentences summarizing each paragraph, and connecting it to the section's argument in a notebook (which is extra work but you'll end up with good recall and can quickly reference notes as well). If it's a regular non-fiction book you could write a quick paragraph at the end of every chapter summarizing what it said and adding your thoughts on it. The act of physically writing and forcing yourself to recall/analyze the material goes a long way for memorizing material. Also, sleep is a big factor in general for memory. If you're having memory problems in general make sure you're getting enough sleep. During sleep your brain is filing things away and needs that time to connect dots. Also you'll just get better as time goes along, so don't worry too much. You're not stupid, maybe it's just been a minute since you needed to recall stuff like that.