I second 'The Care and Keeping of You'. It promotes self respect and good habits, the idea that your body is worthy of care and respect and your health is something you should promote and be responsible for.
I worry about the term 'modesty' even though I dress in a way that would definitely be considered modest and so do the DDc and DS too. I feel that modesty is used and has been used in the past as a way to keep women in their place and not men -- it seems to always apply to women more than men imo. However, back to the subject at hand....
While it's easy to think boys are the enemy and to become focused on issues like modesty (which sort of makes girls wonder if their lives should revolve around what the opposite sex thinks of them), it is far too often girls who cause trouble for other girls, and the sort of trouble that can make a tween's or teen's life a real misery at that. To a large extent boys only figure as pawns in the games girls play with each other. I recommend 'The Smart Girl's Guide to Friendship Troubles', and 'Friends: Making Them and Keeping Them' for friendship issues that come up.
I recommend many titles from the American Girl 'Smart Girl's Guide To...' series -- you can browse through the series on Amazon. My four DDs have benefited hugely from many of them. They made many aspects of their lives in school a lot clearer to them than they would have been otherwise.
'A Smart Girl's Guide to Liking Herself - Even on the Bad Days' puts the focus on management of your own feelings and reiterates the message that you are the only person who can make your life happy or otherwise. Nobody else is going to 'complete' you.
'Stand Up for Yourself!: Every Kid's Guide to Personal Power and Positive Self-Esteem (Dealing with Bullies and Bossiness and Finding a Better Way)' - also by American Girl publishing is a great empowering tool for dealing with cliques, exclusion, taunting, hurtful looks and comments and the little cruelties girls inflict on each other.
'A Smart Girl's Guide to Knowing What to Say' will give your daughter a vocabulary for a lot of different situations, very useful because things come up and tweens don't always respond in a way that moves the situation forward in a positive way.
Having healthy self esteem and assertiveness is important at all ages, not just when puberty hits, and not just in the context of relationships with boys. If you read the books together with your ears wide open you can often find out a lot about the world your daughter or son inhabits.
For you yourself, I recommend 'Queen Bees and Wannabees' by Rosaline Wiseman and a few other books by her along the same lines, such as 'Queen Bee Moms and Kingpin Dads: Dealing with the Difficult Parents in Your Child's Life', and 'Boys, Girls and Other Hazardous Materials'.