Fanged Bunny is right, though the Queen Bees is geared toward an older audience (middle school), it's a really good book that YOU might want to read. I'd look into the book "Little Girls Can Be Mean," which is for girls in elementary grades.
Also, know that it's important to model and discuss healthy boundaries. I don't mean physically (which is also important, but another conversation), but emotionally and socially. She needs to know that it's okay to say no. It's okay if someone's feelings are hurt or they get mad because things didn't go their way. She doesn't have to agree to do something or be bossed around. She can leave a situation. And she should not do things that hurt or make fun of others because someone else thinks it's funny.
Boundaries are hard for younger kids because they're not tangible or visible, and kids at that age are very rules oriented. It's important that she understand that we each have rules for ourselves, to keep from allowing people to cause us to feel bad, or to take advantage of us, or to boss us around. I don't allow people to choose for me whether I will be a part of a group or activity. I choose. You have to teach her how to choose.
I hope that makes sense! :-)
Luck!
ETA: Get used to the phrases "What do you think?" and "Do you think that's true?" and "What could you do?" instead of lecturing her on right and wrong. Often when she considers the situation, she'll come to her own logical conclusions, and understand them better. The goal is for her to learn to navigate these situations without you.