Discuss and refer the book to your friends and family. Discussion will help you discover new opinions on the same topic. Referring will help you become a better teacher. Discover new books through your friends and their bookshelves.
Write notes every single time. Don’t copy paste notes from your kindle or phone, write manually, typing every single word. Also try to paraphrase, this way you are forced to actively think about the material you’re reading.
The best way to retain knowledge is to share it.