What are some of your hacks to retain more information from a book you're reading?

I've started underlining important sentences and plan on skimming the book after I finish it to reread those.

I read like a cup of water. 

Filling myself up 3/4 way or until I feel like I can’t take anymore in. Then I put the book away until the next time I need a drink of water.

Not memorize, internalize.

Reading to feel “overwhelmed” which is normally a few pages or a chapter. The words invoke thoughts or memories that run parallel to “dreams” or experiences. 

The way I internalize them is via a short succinct sentence as a cast or note about the intersection.

So you just sort of memorize everything? What if you read many books?