I know it's horrible - as a librarian - but I LOVE when people write interesting things in books - as an artist.
Some of it's funny, some of it's beautiful.
One time I took an old book of poetry out of my college library, and read it while I was bored in class - I turned a page and a bunch of pen writing jumped out at me - it was some student asking why you were reading this old book, asking if you were bored, and what you were doing right now. I think it had a date on it too. I still regret that I didn't have a camera with me to take a picture of that, and never thought to go get one and do so. I don't know what book it was now.
Leaving it as a note or something inside the book seems kind of a wuss option to me - again, as a librarian, I'd prefer if you wrote your words of wisdom on a removable object. But therein is the issue: the librarian is just going to remove it, so nobody else will ever read it. When you write it in the book itself, it's a little permanent communication with whoever picks it up next - this idea I love, as an artist.
One time as a kid I checked out this book, (which I have since obtained another copy of, for myself, miraculously through my own library book sale, many years later and in a different state; it's amazing, I need to do a post on retro scifi art at some point.) from the kids' section of my library, and on a page with a particularly nasty, Cthuthlu looking monster, it had an arrow pointing to it with a girl's name written in. Middle school drama, man.