I bought my teenage daughter a book that explains all of it in great detail. My mom was very open but there was just still a lot unknown unsaid. Cycle savvy
I’m not sure if you’ll see this, but something I’d encourage you to get a book for her about is fertility awareness. Once she has her cycles, fertility awareness will empower her to understand her fertility, monitor her own health, predict her period, regulate chances of pregnancy down the line, and more. Taking Charge of your Fertility is the classic secular FABM book and here’s a link to the teen version. It’s not my favourite in all ways, but I did read it as a teen and found it a bit helpful. https://www.amazon.ca/Cycle-Savvy-Smart-Teens-Mysteries/dp/0060829648
Beyond that, there’s whole methods with names like FEMM, sympto-thermal, Marquette, Creighton, and more. It’s something not a lot of people really know about yet but just incredibly empowering for most people who menstruate to know this about their bodies, and it’s not something I would anticipate that somebody who doesn’t menstruate would think of talking about!
Cycle Savvy by Toni Weschler gets it really close! (https://www.amazon.com/Cycle-Savvy-Smart-Teens-Mysteries/dp/0060829648)
I suggest pre-reading, as there is some discussion of sex and fertility and conception. It's not lurid, but it is graphic, and you'll be the best judge of whether your kid is ready for that.
No idea about the American Girl book; but I will recommend Cycle Savvy: The Smart Teen's Guide to the Mysteries of Her Body. The author is amazing! THIS is the stuff you should learn in health class.
http://www.amazon.com/Cycle-Savvy-Smart-Teens-Mysteries/dp/0060829648/ref=asap_bc?ie=UTF8
I highly recommend this book
https://www.amazon.com/Cycle-Savvy-Smart-Teens-Mysteries/dp/0060829648
And would avoid anything that leans hard on 28 day cycles as the norm.
A good book for teens is Cycle Savvy: The Smart Teen's Guide to the Mysteries of Her Body.
https://www.amazon.com/Cycle-Savvy-Smart-Teens-Mysteries/dp/0060829648
FEMM Health also offers a TeenFEMM course, though you may need to look for an instructor and ask about enrolling in a course. They don't seem to regularly offer the course through their website. You can find the teacher directory here: https://femmhealth.org/education/teacher-directory/
Here's a bit about learning as a teen from a FEMM educator: https://naturalwomanhood.org/3-myths-about-teaching-teens-about-fertility-awareness-2019/