I too would recommend the Living with Intensity book, I've just finished it and it was probably one of the most useful books I have read regarding giftedness. It covers pretty much everything that has been discussed here - perfectionism, selfhood, self esteem. The Gross book is good too - if a little dry, as mentioned.
I can really relate to the comments so far about authentic challenges. I think it is so hard for girls to really find a sense of themselves in those middle years, especially for gifted kids. The more highly gifted, the harder it is I suspect. Trying to fit in, combined with still not having adequate challenge, even with grade skips, makes it hard to find a) find opportunities for challenge in the first place and b) find a space place where you can engage in authentic challenge and still feel safe.
Good luck and I hope you're looking after yourself too. It's awful to watch someone you love go through something like this.
