I look out for books that have been annotated by one of your kin whenever I'm in a used book store. It's fun to see what other people think is interesting or memorable (or unhinged... Why are you highlighting every occurance of the word "earth"?)
Little notes in the margin can also be a fun plot device, used to great effect in one of the Harry Potter books, (I think?) The Chamber of Secrets.
Writing while reading is a way of focusing on what either resonates with me or confounds me.