I read start to finish. What I've recently started doing, though, is sitting down when I've finished a book and taking about a page of notes. I'll summarize it in a paragraph or so, and then record my impressions, associations, questions, judgments, etc. I've found that knowing I'll be making notes at the end has naturally spurred a deeper level of engagement and made me a more attentive reader. So that's pretty cool.
This is something that I've picked up from Ted Gioia, as explained in this substack post from August:
https://www.honest-broker.com/p/how-i-take-notes. It hasn't been long but so far I've found the practice very fruitful. (I don't mark up books like he does; my library would have some objections.)