Not in library books I might add, but books around the house (all belong to me). I highlight, underline, write in the margins and notes in the back and generally deface them.
I love it when I get books second hand that also have some writing in, or read them at other peoples houses.
It's like old style academic twitter.
Lots of people tell me you should never write in books. #
AIBU? or do you do it too?