I have a vivid memory of an emergency room waiting area (Lizzie had broken a wrist falling from a tree) where Mona sat the whole time with her back to the TV, reading Meet Addy by Connie Porter, another American Girl book. She was in first grade -- I think it was the first chapter book she read on her own. The American Girl series plug along at the store, selling steadily but not amazingly. Each of the American Girl historical figure dolls has an accompanying six book series -- and some have a seventh book focusing on a friend of the central character. The company, which started in 1986, was bought by Mattel (the mega-toy business that brings us Barbie, among many others) 12 years later. They've kept the basic marketing scheme: upscale dolls, clothing and accessories which are sold directly to consumers, and books and a handful of games that are more widely available.
I suspect you'll find more of the "reasonable writing, interesting historical detail" you describe in the Molly book if you guys keep going. They deal with historical issues of whatever time they're about in quiet but straightforward ways. Most of them have the same titles and trajectory in the six books: Meet Molly, Molly Learns a Lesson, Molly's Surprise, etc. One author, Valerie Tripp, wrote the Molly books, as well as all of Kit, Josephina, Felicity and half of Samantha. I will admit that I was disconcerted a few years back when they introduced a historical character who is younger than I: the story of Julie is set in San Francisco in 1974.
The American Girl publications that fly off our shelves, though, are the advice books. They publish quite a few, all aimed at 8-to-12 year olds, with titles like, A Smart Girl's Guide to Friendship Troubles/...to Staying Home Alone/...to her Parents' Divorce/...to Manners/...to Sticky Situations/ ...to Boys. Their advice is pretty solid, full of common sense and pushing girls to think for themselves.
Then there's the runaway bestseller,
The Care and Keeping of You: The Body Book for Girls. It's the ultimate puberty-but-no-sex explainer. We've discussed books about where babies come for young kids, and someday we'll do another entry on puberty and periods. There are other books too, but this one is an excellent place to start.