I just re-read Are You There, God? It's Me Margaret by Judy Blume, which was a favorite book of mine, by my favorite elementary through middle school author, Judy Blume. I probably read it 50 times as a kid. I still really enjoyed it and would really recommend it, especially to a girl who is going through some of the same kinds of "growing up" things that Margaret deals with.

There are details that seem a little old-fashioned, but the same kinds of feelings will always be experienced by "tweens"  (older kids who are not quite "children" but not quite teen-agers..they are in be-"tween")...things like worrying if you are normal or if you are growing up (or growing in the right places!) faster or slower than other kids your age, worrying about getting along with friends, etc. Also Margaret's parents were different religions when they were married...Jewish and Christian...and she doesn't know where she fits into all that....whether she's one or the other, both or none.  

For a book about the same kinds of things, but from a boy's perspective...try Then Again, Maybe I Won't (another favorite of mine by Judy Blume.)