There’s a great book that may not be appropriate for all girls, but is good if you’re in the outside world and struggling with modesty. This book helped me more than all the other tznius books I read (except Oz Vhadar Lvusha which I liked for its hashkafic strength)
It’s called “Doesn’t Anyone Blush Anymore?” by Manis Friedman, and it’s very good for being able to appreciate and explain tznius to people who are not from a frum background.