L.B.
I am the tech specialist for my school district. The state technology standards, at least here, include communicating with others, which includes the use of email. If you are concerned about what she is writing, I think you are on the right track with talking with her about what is okay and what is not okay. These discussions should be happening in class also. I'm not sure that the email she sent is what would be considered okay for school email- as it is supposed to be an extension of class activities and assignments (This is a teacher responsibility- the guidelines may need to be explained better in class by the teacher as far as what they are supposed to use their school email accounts for and what they are not for). As far as a book, I would recommend "Raising a Digital Child" by Mike Ribble. It is written for parents. You can get it from Amazon for about $20. There are also a couple of websites that can give you and your daughter something to talk about- www.netsmartz.org, www.connectsafely.org, and http://cybersmartcurriculum.org/
A very important tip- don't make your daughter scared to go online, just teach her how to behave online and what to do (a response you think is appropriate) if someone says or does something she doesn't like. Scare tactics have been proven not to work, as children, especially teens, will do it anyway. There has been quite a bit of success in teaching kids how to act appropriately online and how to respond appropriately to others, including proper email etiquette.