I bought this for my seven-year-old daughter, who's been having issues at school, and I've read it out loud to her in installments.
It's aimed at 9+ years in content, being written in that cool-clothes-and-cupcakes style that you find in magazines for tweenies and early teens, and it is decorated in lots of funky pastels, but beneath all that, it actually contains a lot of common sense. And how female friendships work is applicable whether you are seven or seventeen (or seventy.) I found it immensely useful, because the author has tried to anticipate every single friendship problem that could ever possibly occur in a young girl's life (cliques, rivalries, being left out and lonely, fall-outs, being friends with boys etc. etc.), and then has written it all out in friendly little bite-sized chunks that are very easy for a child to read and absorb.
My daughter discovered that she is not the only girl in the world to be having these experiences, that the author was backing up everything that I had been saying, and that friendship is a skill that can be learnt and practiced same as anything else. And a skill to develop for life. There's also advice to the child on building up their self-esteem and confidence, and pursuing their passions and interests, so they are much more appealing to potential friends, and can survive if there are no friends around at any particular time.
There are some contact numbers and websites at the end for more serious problems like bullying. They are all British contacts.
I bought this for my nearly 8 year old daughter, as I was fed up with saying the same things over & over to her when she'd come home from school upset by her 'friends'. As a reluctant reader this has been the first book she voluntarily went off to read, which in itself was a good thing, but the book basically gives sensible advice in a way that she could understand - there is a section on cliques, on how friendships can come & go, and even on the difference between a friend who is a boy and a 'boyfriend'. Would recommend for junior age girls. I have bought some of the 'American Girl' series of advice books for her now, as this seems to be a one off book and not a series.
My daughter is going to Senior school in September and was very worried about making new friends and how to do this. This books is full of tips and advice presented in an easy to digest format. Her view is 'its brilliant'.