I think one of the best tips is to search for the summary or the main idea or a A-HA moment that finally connects concepts and ideas. I never highlighted because I like to have one pen in the moment so I usually signaled that A-HA by boxing it or something extra special. It also might be helpful going between the books by marking where ideas relate to other ideas in the book. A quick index so you can open all those books easily all at once if you need to see all the information together
For language you might also wanna have a system of compiling words you’ll actually use regularly. You might not use “rocket ship” daily but you might use “milk”. Writing those words in the margins can be helpful so when you’re flipping through pages your eyes catch them easily
Just don’t overcomplicate your system. Otherwise you spend more time trying to be matchy-matchy than you do actually absorbing info (I say this as a person who has a massive pencil case of stickers and inks and highlighters and tabs that I’ve spent an unholy fortune on)