For you girls, I suggest... For Young Women Only.
For Young Women Only is an insightful book on how guys think. The authors have taken surveys of guys from ages 15-21 and have had small groups with guys of that age range. The book talks about what guys wish girls knew about them. It's very informative and helps girls to understand the mystery of the male mind. It can help you with dating relationships, friendships, and even relationships with your annoying brother. I greatly recommend it for any girl who wants to know how guys think.
And... Sexy Girls: How Hot is Too Hot?
I've not finished this book, and it doesn't have a lot of relevance to me because I'm so completely *coughoverthetopcough* modest. But if you have modesty issues (whether you know it or not) you should definitely read this book.
For you guys I suggest... For Young Men Only.
Basically the same thing as For Young Women Only but for guys. I haven't read it, obviously. =P