when you're 8 years old and in 3rd grade and your best friend starts developing breasts (nonfiction division): The Period Book.
It's intensely reassuring, a lot about how it doesn't mean you have to grow up, exactly, yet. Some readers may need help with it at age 8 (and how crazy is it that these things are happening before a lot of kids are good readers?) but the tone and the clear information are worth it.
marcialerner.com 
What about It’s Perfectly Normal by Robie Harris and Michael Emberly? We are about 8 years behind you (god willing) and have been reading It’s Not the Stork (by the same team). So I can’t speak to how heteronormative it is; I guess there is a full chapter on homosexuality. I do know that they conflate sex and gender sometimes, in both the little kid book and probably the teenage book.
If the teenage version is as sensitive as the one for little kids it might do the trick.
LikeLike