I used to be one of those people who were all precious about their books and thought writing in them was akin to sacrilege, but... it's just paper. It's the contents that are important, and if any kind of physical interaction with a book helps the owner have a better and more profound reading experience, then the book is serving its purpose no matter what it looks like. And unless you are intending from the start to pass the book on to someone else, it should be yours to do with as you please.
We all know Ruby would never donate that book anyway, it will sit on her bookshelf gathering dust until the next time she rearranges her room, so she might as well exercise some critical thinking and make some notes on it.