Best way to retain info when reading a textbook?

Use it some how. Or teach it to someone.

Write down a page of notes per chapter or more.

Active reading. So read with a purpose in mind. For instance, to answer a specific question. And teach what you’re reading to someone else.

Take notes but do something with them. Perhaps order them into themes, create a mind map or a table and look for relationships to other reading you’re doing. This not only helps you to analyse what you’re reading and to connect the various pieces of whatever it is you’re trying to understand, it helps you to retain information.

As to teaching, what I mean is after you’ve read and taken notes, explain what you’ve read to your mum, wife, dog, cat… doesn’t matter who. Tell them what the reading means in terms of the topic you’re studying. Who was the author? Why do they matter? What was their argument? This again will not only help you to retain the info, it will help you to understand it and evaluate it. If you can’t explain it well, you know to go back and work with it again.

Don’t just read. That’s very inefficient. What are you studying?

Summarize each paragraph/section/chapter on a post-it note in your own words.

Mentally prepare to give a talk about the subject matter.